What is the quarts "range" on the dipstick on these trucks?

[jfoj]

@Vladimir2306

I think the point you are missing is fuel contamination is like a Yo-Yo. Depending on how the vehicle is used, the outside ambient temperatures and the season you are currently in, the fuel dilution percentage is always a moving target. Make a bunch of 10-20 minute drives in the Winter and never fully warming the oil up, fuel percentages will be high, then take a 1-2 hours highway trip, the fuel can be "cooked" out of the crankcase, maybe not all the fuel, but the percentage will be reduced. Adding make up oil for oil consumption will CLEARLY reduce the fuel dilution percentage. Use the vehicle mainly for 45+ minute trips, fuel contamination will be a much smaller problem then remote starting the vehicle and allowing it to warm up at idle and making a bunch of trips between 10-20 minutes at a time. Once you change the oil, you RESET the baseline and have very little fuel contamination, probably a small amount of residual oil with fuel left over in the oil cooler, oil cooler lines and some small amount that has not drained out of the engine.

During these times of higher fuel contamination, the fuel in the oil will reduce the oil viscosity and any safety margin will be lost. Given the bordeline protection 0W20 oil provides for these engines they way they are loaded on the highway, fuel dilution is a factor that may not be as much of a problem for smaller engines and smaller vehicles.

Case in point I had a 1987 Buick Grand National Turbo 6 cylinder that cruised at 70 MPH at 1800 RPM on the highway. BUT this car was 1/2 the weight of these trucks and far more areodynamic, the transmission was a 4 speed automatic with only 1 Overdrive gear. These engines ran 10W30 engine oil and had no issues running lower RPMs on the highway because the engine was not constantly hittig 80%+ loading on a regular basis. Probably running at an average of 50% loading, I have no data on this vehicle because OBDII was in its early stages and there were not low cost ways to aquire data. Also understand if this engine went to 100% loading on the highway, the vehicle could go from 50-70 MPH in under 2 seconds!

As for your concern that a slightly higher oil viscostity will not make its way to the bearings given the oil passage size, this is a non issue. There are no magical smaller oil passages or overly tight bearing clearances. Bearing clearances have not signifigantly changed for years. And again, the exact same engine platform has different specs for oil viscosity depending on the application. Part of the "application" is how many of the specific platforms are being sold, nothing to do with performance, protection or reliability. While you are not in the US and did not grow up in the US you may not understand the stupid rules and restrictions that non techinical goverment employees put on automotive industry for the past 50 years.

BUT as of this week, this is in the process of changing for the better! If we are lucky, DOD/DFM may be a thing of the past.

 

[blanchard7684]

Thanks for sharing these equations. Very educational.

I was just about to say the bearing clearance is a big component in all this and I like the way they used to do back in the day where oil viscosity was recommended in the manuals based on ambient temp and use conditions. Constant high temp and loaded applications called for higher viscosity for sure. Here, running on 2 cylinders may be construed as a high load scenario perhaps??

Anyone here have access to factory service manuals? That ought to clarify things a bit more

Examples from old GM manuals

View attachment 452416

My Tundra manual looks alot like that--exceptions for 0w20 based on ambient and load.

 

[StephenPT]

Here's a chart that shows recommended oil viscosity based on bearing clearance and engine oil operating temperature. Not sure what the main bearing clearance spec is for the L87, but this gives you an idea of how tighter clearances and engine operating temperature play hand in hand when specifying oil viscosity.

What I Learned Today — Bearing Clearances vs. Oil Viscosities

Choosing your oil viscosity based on bearing clearances can be challenging. Luckily Jeff Smith adds some insight into the problem.

www.enginelabs.com

 

[jfoj]

These are pretty typical bearing cleareances that have been somewhat the standard for many years. Usually oil cleareance is based on journal diameter as outlined in the article.

I have already logged 226F engine oil temp with nothing but a full tank of fuel and a single driver in the vehicle at highway cruise at approximately the 2 hour mark at 64F ambient outside temperature. I have yet to make one of my 6 hour runs at 100F ambient and the road at between 130-140F after baking in the direct sun all day. So I expect that I may hit 230-240F?

So this is part of the concern with 0W20 and the excessive engine loading while cruising on the highway pulling very slight grades. Add fuel in the oil, you know the drill, or at least wise folks know the drill.

Next is to check Towing mode out a bit more and see if the front shutters open and if there is any signifigant impact on the engine cooling temperature or oil temperature.

 

[Vladimir2306]

These are pretty typical bearing cleareances that have been somewhat the standard for many years. Usually oil cleareance is based on journal diameter as outlined in the article.

I have already logged 226F engine temps with nothing but a full tank of fuel and a single driver in the vehicle at highway cruise at approximately the 2 hour mark at 64F ambient outside temperature. I have yet to make one of my 6 hour runs at 100F ambient and the road at between 130-140F after baking in the direct sun all day. So I expect that I may hit 230-240F?

So this is part of the concern with 0W20 and the excessive engine loading while cruising on the highway pulling very slight grades. Add fuel in the oil, you know the drill, or at least wise folks know the drill.

Next is to check Towing mode out a bit more and see if the front shutters open and if there is any signifigant impact on the engine cooling temperature or oil temperature.

Engine temperature 224F? Are you confusing it with oil temperature? Our engine is cold, compared to 6.2 on K2. There the operating temperature was 100C, 212F, and on T1 the operating temperature was 93-95C, that's 200-203F. 224F engine temperature is a lot, it could be engine overheating.

 

[jfoj]

Engine temperature 224F? Are you confusing it with oil temperature? Our engine is cold, compared to 6.2 on K2. There the operating temperature was 100C, 212F, and on T1 the operating temperature was 93-95C, that's 200-203F. 224F engine temperature is a lot, it could be engine overheating.

Good catch. No confusion, was supposed to be Engine Oil Temp, my mind was operating faster than my fingers. Corrected the prior post.

The problem is this vehicle is even optioned with the Max Trailer Towing package which is has a larger radiator, but unclear if the internal oil cooler is any larger or more efficient than the standard radiator. Interested to see what happens to the oil temp temp once the outside ambient get in the upper 90F to 100F and the asphalt roadway is in the 130F range after baking in the sun all day and I am driving for 6 hours straight.

I do not have one of these trips schueduled yet, but in the next few months the weather conditions and ambient temp will be getting higher so I will likely have data for at least a 2 hour plus drive to see what happens with the oil temp.

As for Engine Coolat Temperature, I think the current genration vehicles operate a bit higher in temperature than you state. While I have seen in the low 200F's around town, on the highway, this is the graph for the same run where my Oil Temp topped at 226F.

The average Coolant Temp was around 207F with a peak around 213F.

 

[Vladimir2306]

Good catch. No confusion, was supposed to be Engine Oil Temp, my mind was operating faster than my fingers. Corrected the prior post.

The problem is this vehicle is even optioned with the Max Trailer Towing package which is has a larger radiator, but unclear if the internal oil cooler is any larger or more efficient than the standard radiator. Interested to see what happens to the oil temp temp once the outside ambient get in the upper 90F to 100F and the asphalt roadway is in the 130F range after baking in the sun all day and I am driving for 6 hours straight.

I do not have one of these trips schueduled yet, but in the next few months the weather conditions and ambient temp will be getting higher so I will likely have data for at least a 2 hour plus drive to see what happens with the oil temp.

As for Engine Coolat Temperature, I think the current genration vehicles operate a bit higher in temperature than you state. While I have seen in the low 200F's around town, on the highway, this is the graph for the same run where my Oil Temp topped at 226F.

The average Coolant Temp was around 207F with a peak around 213F.

You won't have anything in hot weather, well, maybe the temperature level will be at the Upper Limit. 200-210F for the coolant temperature and 230-240F for the oil temperature, but this is a normal temperature for both. I drove in the heat with a trailer weighing about 1.1-1.2 tons.

 

[jfoj]

So if we take the Engine Labs article that StephenPT linked about oil viscosity and make the following observations and assumptions and then we back into the answer a bit.

For the main bearing oil clearance for an aluminum block and the recommended oil viscosity.

1. Assume we do not know or care about the L87 main bearing oil clearances.

2. We use the reference point in the chart for an aluminum block at the mid oil temperature range (160-220F) and find where 0W20 oil is recommended. This is for main bearing clearances is in the range of 0.0012-0.0017 inches.

3. Then we look at the same bearing clearance row, but look at the oil viscosity recommendaton for the next oil temperature range of over 220F oil temperature, we see the recommended oil viscosity rating is 5W30 or 10W30.

Then we do the same thing for the steel connecting rods.

1. Assume we do not know or care about the L87 rod bearing oil clearances.

2. We use the refernece point in the chart for steel connecting rods at the mid oil temperature range (160F-220F) and find where 0W20 oil is recommended. This is for rod bearing clearances is in the range of 0.0016-0.0021 inches.

3. Then we look at the same bearing clearance row, but look at the oil viscosity recommendation for the next oil temperature range over 220F oil temperature, we see the recommended oil viscostiy rating is 5W30 or 10W30.

My initial high oil temperature log indicated a 226F max with no real load and cooler ambient tempeture during a 2 hour high cruise. Vladimir indicated he has seen oil temps in the 230-240F range towing something relatively small. I expect to see oil temps in this range on long drives with vehicle little weight in the vehicle as well.

So this leads to the assumption between where the oil temps the 6.2l is operating at at highway speeds and the engine load percentages the 6.2l sees on the highway regularly with minimum weight in the vehicle or not towing a trailer, using 0W20 oil leave ZERO safety margin.

Add to this operating on less than 4 cylinders during DFM operation, Fuel diluted oil and Low Speed Pre Ignition, we wonder why 6.2l are having bearing failures.

 

[Vladimir2306]

So if we take the Engine Labs article that StephenPT linked about oil viscosity and make the following observations and assumptions and then we back into the answer a bit.

For the main bearing oil clearance for an aluminum block and the recommended oil viscosity.

1. Assume we do not know or care about the L87 main bearing oil clearances.

2. We use the reference point in the chart for an aluminum block at the mid oil temperature range (160-220F) and find where 0W20 oil is recommended. This is for main bearing clearances is in the range of 0.0012-0.0017 inches.

3. Then we look at the same bearing clearance row, but look at the oil viscosity recommendaton for the next oil temperature range of over 220F oil temperature, we see the recommended oil viscosity rating is 5W30 or 10W30.

Then we do the same thing for the steel connecting rods.

1. Assume we do not know or care about the L87 rod bearing oil clearances.

2. We use the refernece point in the chart for steel connecting rods at the mid oil temperature range (160F-220F) and find where 0W20 oil is recommended. This is for rod bearing clearances is in the range of 0.0016-0.0021 inches.

3. Then we look at the same bearing clearance row, but look at the oil viscosity recommendation for the next oil temperature range over 220F oil temperature, we see the recommended oil viscostiy rating is 5W30 or 10W30.

My initial high oil temperature log indicated a 226F max with no real load and cooler ambient tempeture during a 2 hour high cruise. Vladimir indicated he has seen oil temps in the 230-240F range towing something relatively small. I expect to see oil temps in this range on long drives with vehicle little weight in the vehicle as well.

So this leads to the assumption between where the oil temps the 6.2l is operating at at highway speeds and the engine load percentages the 6.2l sees on the highway regularly with minimum weight in the vehicle or not towing a trailer, using 0W20 oil leave ZERO safety margin.

Add to this operating on less than 4 cylinders during DFM operation, Fuel diluted oil and Low Speed Pre Ignition, we wonder why 6.2l are having bearing failures.

However, I drive 0-20 on oil without any problems, and I have 75 thousand miles, I drive fast. Fast means around 90-105 mph on the highway. or with a trailer at 80-90 mph. and I change the oil every 7500 miles. And this is the worst mode for 0-20 in your hypothesis. And I have no problems, neither with the oil, nor with the 6.2 engine.

 

[KMeloney]

I'd like to chime in and also state that I put the dipstick up to max range and that is typically at least 8.5 quarts with a full filter.

Oil viscosity is related to rotor eccentricity. Too much eccentricity and you have metal to metal contact. Too little eccentricity and you get high bearing temps and vibration problems (the kind you don't feel in your seat but will wreck a crank and bearings nonetheless).

This is all connected via the Sommerfeld relation:

View attachment 452414

Eccentricity term is on right hand side. Sommerfeld number is on left hand side.

u is viscosity, omega is rpm, W is load, L is bearing width, R is bearing radius, C is bearing clearance.

You can see there is an L^3/C^2 term here where eccentricity is suuuper sensitive to clearance and width. This isn't an issue in this discussion about viscosity.

What is left is viscosity, Load W, and RPM.

Through iterative approaches the following can be said.

Any term in numerator that increases will reduce eccentricity.
Any term in denominator that increases will increase eccentricity.

So...by only increasing rpm, the eccentricity is reduced. Therefore reducing viscosity will counteract this and induce eccentricity. So if your application is constant high rpm it would actually favor lower viscosity. Oil cooling will need to be tightly controlled to keep this situation under control.

Increasing load will increase eccentricity. Therefore increasing viscosity will counteract this and reduce the eccentricity. So if your application is for towing or accelerating uphill or a super heavy SUV, the load is high at every RPM. Therefore, this would favor higher viscosity to reduce eccentricity.

Worst case is obviously high loads and low rpms. This would give maximum eccentricity. The oil wedge reaction force against this is dependent on the viscosity.

View attachment 452415



This is a "use case" that is very typical of SUVs and trucks but also drag racing. So with high loads and low rpm the viscosity must be higher to keep eccentricity managed.

Downside to running viscosity too high (SAE 50+) in these situations is cold oil...therefore the mutli viscosity grades are helpful in giving a range of viscosities keep eccentricity from getting too low and causing vibration if there is extended low load, high rpm situations while oil is cold.

Thank you. So, what oil do you recommend running in the 6.2?

 

[KMeloney]

So if we take the Engine Labs article that StephenPT linked about oil viscosity and make the following observations and assumptions and then we back into the answer a bit.

For the main bearing oil clearance for an aluminum block and the recommended oil viscosity.

1. Assume we do not know or care about the L87 main bearing oil clearances.

2. We use the reference point in the chart for an aluminum block at the mid oil temperature range (160-220F) and find where 0W20 oil is recommended. This is for main bearing clearances is in the range of 0.0012-0.0017 inches.

3. Then we look at the same bearing clearance row, but look at the oil viscosity recommendaton for the next oil temperature range of over 220F oil temperature, we see the recommended oil viscosity rating is 5W30 or 10W30.

Then we do the same thing for the steel connecting rods.

1. Assume we do not know or care about the L87 rod bearing oil clearances.

2. We use the refernece point in the chart for steel connecting rods at the mid oil temperature range (160F-220F) and find where 0W20 oil is recommended. This is for rod bearing clearances is in the range of 0.0016-0.0021 inches.

3. Then we look at the same bearing clearance row, but look at the oil viscosity recommendation for the next oil temperature range over 220F oil temperature, we see the recommended oil viscostiy rating is 5W30 or 10W30.

My initial high oil temperature log indicated a 226F max with no real load and cooler ambient tempeture during a 2 hour high cruise. Vladimir indicated he has seen oil temps in the 230-240F range towing something relatively small. I expect to see oil temps in this range on long drives with vehicle little weight in the vehicle as well.

So this leads to the assumption between where the oil temps the 6.2l is operating at at highway speeds and the engine load percentages the 6.2l sees on the highway regularly with minimum weight in the vehicle or not towing a trailer, using 0W20 oil leave ZERO safety margin.

Add to this operating on less than 4 cylinders during DFM operation, Fuel diluted oil and Low Speed Pre Ignition, we wonder why 6.2l are having bearing failures.

What are the mechanics/physics behind warming the truck up for a considerable time and then driving for a short time that causes the oil to be thinned with gas? Why does this allegedly happen (or happen to a greater extent) under these circumstances? What's going on in the engine that's promoting the dilution?

 

[blanchard7684]

Thank you. So, what oil do you recommend running in the 6.2?

5w30 or 0w40. 0w40 if you are towing alot, or driving in high ambients frequently, or have to drive up steep grades often (and pass other vehicles).

I think you can get away with 0w20 in the 5.3 if you opt for a more frequent oil change interval. I've been running 0w20 in my 2024 5.3 Suburban because I bought a bunch of it. I have 8000 miles and three oil changes though...

 

[jfoj]

However, I drive 0-20 on oil without any problems, and I have 75 thousand miles, I drive fast. Fast means around 90-105 mph on the highway. or with a trailer at 80-90 mph. and I change the oil every 7500 miles. And this is the worst mode for 0-20 in your hypothesis. And I have no problems, neither with the oil, nor with the 6.2 engine.

@Vladimir2306

It seems you drive you vehicle very differently than many of us here in the USA. There are few places people can drive constantly at 90-105 MPH on the highway without being quickly followed up by traffic enforcement and at these speeds you can and will lose your drivers license and the insurance prices may increase and/or you may be canceled. I woudl say most drivers in the US depending on the local posted speed limit may only travel 5-10 MPH higer than than the posted speed. So this typically at most wil be maybe 80 MPH, more often I would expect the "Average" speed most are traveling on the US roads is between 65-75 MPH because many Interstate posted speed limits are 55-65 MPH, some are higher but it is more "State" dependant as there is really only a "National" 55 MPH mininum speed limit these days. Many years ago, the US had a "National" 55 MPH speed limit maximum.

So this being said, a the engine RPM you are operating at is in a better range with less low RPM "lugging". Additionally with the higher RPM and oil temperatures you are likely burning off any fuel contamination in the oil.

I also assume at these speeds and higher RPM that the DFM may be less likely to function unless you have it bypassed. Would also be interesting to see if you are triggering the 2nd stage in the oil pump on a regular basis at these speeds.

Do you typicaly use Trailer Towing mode when you drive at these speeds or when you are towing your trailer?

I also recall you mentioned that you had no problem getting Premium fuel in Russia, is this correct and what Octane rating is typically available?

Do you have any data for these 90-105 MPH drives that you can share?

 

[viven44]

However, I drive 0-20 on oil without any problems, and I have 75 thousand miles, I drive fast. Fast means around 90-105 mph on the highway. or with a trailer at 80-90 mph. and I change the oil every 7500 miles. And this is the worst mode for 0-20 in your hypothesis. And I have no problems, neither with the oil, nor with the 6.2 engine.

Just curious. What are the average high temps where you live ?

Just like any thermodynamic system, total temp = ambient temp + temp rise from power dissipation. Down here in TX where summer days are 100F and warmer a couple of months, on my old trucks I definitely use the higher viscosity option.

 

[blanchard7684]

What are the mechanics/physics behind warming the truck up for a considerable time and then driving for a short time that causes the oil to be thinned with gas? Why does this allegedly happen (or happen to a greater extent) under these circumstances? What's going on in the engine that's promoting the dilution?

DI engines spray fuel directly into the combustion chamber. When engine is cold, the "atomization" or mixing with air is or can be very poor. This leads to fuel droplets agglomerating especially on cold surfaces like cylinder walls. This will mix with lube oil and overtime create a fuel dilution in lube oil. It is a slow process but one that can take shape over an oil change interval of 7000 or more miles.

Combustion chamber design is set to cause swirl, squish, and quench. This is to promote mixing of fuel and air to be as uniform of a mix as possible. Swirl is circumferential motion, squish is radial motion, and quench is degree of uniformity vertically.(It also refers to clearance between head and piston at TDC.)

My opinion only--I think high swirl designs promote the fuel agglomeration on cylinder walls...

The contrast is with older port fuel injection (or carburetors) that mix fuel and air upstream of combustion chamber. The mixing is almost perfect.

Then of course fuel is a solvent for lube oils just based on the chemistry. So it will disrupt the "film" properties and essentially reduce the reaction force capabilities of the oil wedge.

So the more viscosity you start with, the more you can tolerate reductions before your eccentricity hits 1.0 which is technical metal to metal contact every revolution.

 

[Vladimir2306]

@Vladimir2306

It seems you drive you vehicle very differently than many of us here in the USA. There are few places people can drive constantly at 90-105 MPH on the highway without being quickly followed up by traffic enforcement and at these speeds you can and will lose your drivers license and the insurance prices may increase and/or you may be canceled. I woudl say most drivers in the US depending on the local posted speed limit may only travel 5-10 MPH higer than than the posted speed. So this typically at most wil be maybe 80 MPH, more often I would expect the "Average" speed most are traveling on the US roads is between 65-75 MPH because many Interstate posted speed limits are 55-65 MPH, some are higher but it is more "State" dependant as there is really only a "National" 55 MPH mininum speed limit these days. Many years ago, the US had a "National" 55 MPH speed limit maximum.

So this being said, a the engine RPM you are operating at is in a better range with less low RPM "lugging". Additionally with the higher RPM and oil temperatures you are likely burning off any fuel contamination in the oil.

I also assume at these speeds and higher RPM that the DFM may be less likely to function unless you have it bypassed. Would also be interesting to see if you are triggering the 2nd stage in the oil pump on a regular basis at these speeds.

Do you typicaly use Trailer Towing mode when you drive at these speeds or when you are towing your trailer?

I also recall you mentioned that you had no problem getting Premium fuel in Russia, is this correct and what Octane rating is typically available?

Do you have any data for these 90-105 MPH drives that you can share?

It is clear that I drive completely differently than the average American)) Fortunately, our toll roads in Russia allow us to move between cities quickly and comfortably.
The photo shows the revs at a speed of 170 km/h, which is about 105 miles per hour
But you just said above that it is fast driving under load and in the heat that kills the 0-20 oil. Well, it doesn't kill anything. The oil works fine. I repeat, I have 75k miles, and I change the oil every 7500 miles, without resorting to early oil changes. The only thing is that I change the fluid in the automatic transmission and transfer case and axles every 30K miles.
Yes, and I fill up with petrol grade 95 in Russia, which is about 91 according to the American standard. So there is nothing unusual, I just follow the regulations prescribed by the manufacturer.

 

[Vladimir2306]

Just curious. What are the average high temps where you live ?

Just like any thermodynamic system, total temp = ambient temp + temp rise from power dissipation. Down here in TX where summer days are 100F and warmer a couple of months, on my old trucks I definitely use the higher viscosity option.

I live in Moscow, Russia, it is a city with a moderate climate, in the summer we have up to 80+95F, in the winter it can be up to -22F. At the same time, I travel a lot around our country, visiting regions in the winter where it can be -50F, and in the summer I go to the Southern regions where 100-115F is not uncommon))) but my main place of residence is, as I said above, a region with a moderate climate.

 

[KMeloney]

DI engines spray fuel directly into the combustion chamber. When engine is cold, the "atomization" or mixing with air is or can be very poor. This leads to fuel droplets agglomerating especially on cold surfaces like cylinder walls. This will mix with lube oil and overtime create a fuel dilution in lube oil. It is a slow process but one that can take shape over an oil change interval of 7000 or more miles.

Combustion chamber design is set to cause swirl, squish, and quench. This is to promote mixing of fuel and air to be as uniform of a mix as possible. Swirl is circumferential motion, squish is radial motion, and quench is degree of uniformity vertically.(It also refers to clearance between head and piston at TDC.)

My opinion only--I think high swirl designs promote the fuel agglomeration on cylinder walls...

The contrast is with older port fuel injection (or carburetors) that mix fuel and air upstream of combustion chamber. The mixing is almost perfect.

Then of course fuel is a solvent for lube oils just based on the chemistry. So it will disrupt the "film" properties and essentially reduce the reaction force capabilities of the oil wedge.

So the more viscosity you start with, the more you can tolerate reductions before your eccentricity hits 1.0 which is technical metal to metal contact every revolution.

Fantastic explanation. Thank you!

Now, I was always told to "warm up the engine" prior to driving so that the oil had a chance to lube everything prior to high-RPM operation -- at that THIS is how you prolong the life of an engine. Is the reasoning behind NOT doing this that the engine doesn't actually get warmed up this way, or this provides a good amount of time for fuel agglomeration, or only actually driving the truck/getting into higher RPMs is what warms the engine sufficiently, or...?

Do YOU have any ideas as to why GM wouldn't at least recommend higher-viscosity oil (via a bulletin or something) at this point as a measure toward preventing engine failure?

 

[Vladimir2306]

Fantastic explanation. Thank you!

Now, I was always told to "warm up the engine" prior to driving so that the oil had a chance to lube everything prior to high-RPM operation -- at that THIS is how you prolong the life of an engine. Is the reasoning behind NOT doing this that the engine doesn't actually get warmed up this way, or this provides a good amount of time for fuel agglomeration, or only actually driving the truck/getting into higher RPMs is what warms the engine sufficiently, or...?

Do YOU have any ideas as to why GM wouldn't at least recommend higher-viscosity oil (via a bulletin or something) at this point as a measure toward preventing engine failure?

Great question, maybe because GM knows that the cause of engine failure is not oil?))

 

[blanchard7684]

Fantastic explanation. Thank you!

Now, I was always told to "warm up the engine" prior to driving so that the oil had a chance to lube everything prior to high-RPM operation -- at that THIS is how you prolong the life of an engine. Is the reasoning behind NOT doing this that the engine doesn't actually get warmed up this way, or this provides a good amount of time for fuel agglomeration, or only actually driving the truck/getting into higher RPMs is what warms the engine sufficiently, or...?

Do YOU have any ideas as to why GM wouldn't at least recommend higher-viscosity oil (via a bulletin or something) at this point as a measure toward preventing engine failure?

Speculative ideas

1) recommendation may be forthcoming
2) They may be looking at multiple variables: engine mapping, transmission mapping, tolerance stacking across a large range of parts before they conclude a change in oil type is a necessity. It will be the last thing they do because it will affect overall fuel economy and that is a huuuge deal in the world of an oem. If they can do an over-the-air update to transmission shifting or engine mapping to keep the engine out of a low rpm&high load situation that is easier than changing oil type.