I can confirm that the G80 is a clutch based system -- just had them replaced in my daughter's '03 Sierra -- in case that helps.
Limited slip additive or not?
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I can confirm that the G80 is a clutch based system -- just had them replaced in my daughter's '03 Sierra -- in case that helps.
Jolly Roger
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This is not argued. Its the fact that it is not to be ran with a friction modifier that has people up in arms..
Ok I am now officially irritated. I have always used AC Delco 12378261 which now comes up as 88900401. Last time I decide I wanted to use Royal Purple because it’s the best, right? So I used their 75-90 synthetic gear oil. I have read it’s good for the G80. I didn’t look at the label obviously. Their website stated it’s GL-5 and GL-4 spec’d. The damn bottles state at the bottom that it has you know what. Grrr. It’s about $100 between the 3 quarts of oil and the gasket. I have not noticed any issues with the diff. I’m now going into the rabbit hole to see if the Royal Purple is ok. If I’m not back in 3 days send help. Probably should be a shrink and an anger management specialist.
Jolly Roger
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With an issue so debated online then my default answer is to change the oil more often than they recommend and that in itself is probably a better indicator of life expectancy than whether or not the additive is in the oil.
Considering professionals disagree then there truly may not be a right answer but I would err on the side of Eaton and not GM.
Considering professionals disagree then there truly may not be a right answer but I would err on the side of Eaton and not GM.
I’m leaving the RP in mine. I have read enough. There’s plenty of guys that run it with no issues as well as Amsoil or Mobil 1 which all have FM. Seems the worst that can happen is chatter, at which point I would change the fluid to something else. I think GM just wants to sell their product for twice the price as anyone else. I read you just need the GL5 spec and just don’t add ADDITIONAL FM. Here’s what an Eaton expert has to say:
What Eaton says. "From Mr. Ralph Holmquist of Eaton, the maker of the locking differential: "The maintenance schedule for the rear axle was developed by American Axle & Manufacturing and GM truck based on multiple tests. The Eaton locker does not require additional maintenance nor does it add heat to the lube. The lube will darken due to the carbon wear on the clutch surfaces, much the same as a disc brake pad & rotor. This does not damage axle components such as seals or bearings. However, a new axle can produce excessive temperatures (plus 350 degrees F) due to the ring & pinion breaking in that will break the lube additives down. Avoid high loads, trailer towing and high speed extended driving during the initial break in of the vehicle. After the break in period axle temps will level at a much lower figure. Lube changes are a good idea because the additives are replenished and contaminates such as casting sand are eliminated. The axle is filled at the factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276. The GM service # is 12378261. This is the only lube we have done extensive testing with to insure locker compatibility. The only negative to using one of these other lubes is an increased potential for clutch chatter. This really doesn't hurt anything and can be corrected by changing the lube. I noticed in the latest GM owners manual the term "or equivalent" when referring to the lube specification. Look for a GL5 rating on the bottle to make sure the ring & pinion, seals & bearings are protected. Limited slip additive is not needed." The following information outlines the type of rear axle lubrication utilized in GM Truck axles. I. Fluid Type II. The 1500, 2500 and 3500 GMT 800 Trucks utilize SAE 75W-90 Synthetic Axle Lubricant. The GM part number is 12378261 and the specification is 9986115.
Edit: I also read that GM refuses to provide the exact specifics of “specification 9986115“ to the other fluid manufacturers probably because it doesn’t mean anything and they just want to corner the market on the oil and charge whatever they want.
What Eaton says. "From Mr. Ralph Holmquist of Eaton, the maker of the locking differential: "The maintenance schedule for the rear axle was developed by American Axle & Manufacturing and GM truck based on multiple tests. The Eaton locker does not require additional maintenance nor does it add heat to the lube. The lube will darken due to the carbon wear on the clutch surfaces, much the same as a disc brake pad & rotor. This does not damage axle components such as seals or bearings. However, a new axle can produce excessive temperatures (plus 350 degrees F) due to the ring & pinion breaking in that will break the lube additives down. Avoid high loads, trailer towing and high speed extended driving during the initial break in of the vehicle. After the break in period axle temps will level at a much lower figure. Lube changes are a good idea because the additives are replenished and contaminates such as casting sand are eliminated. The axle is filled at the factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276. The GM service # is 12378261. This is the only lube we have done extensive testing with to insure locker compatibility. The only negative to using one of these other lubes is an increased potential for clutch chatter. This really doesn't hurt anything and can be corrected by changing the lube. I noticed in the latest GM owners manual the term "or equivalent" when referring to the lube specification. Look for a GL5 rating on the bottle to make sure the ring & pinion, seals & bearings are protected. Limited slip additive is not needed." The following information outlines the type of rear axle lubrication utilized in GM Truck axles. I. Fluid Type II. The 1500, 2500 and 3500 GMT 800 Trucks utilize SAE 75W-90 Synthetic Axle Lubricant. The GM part number is 12378261 and the specification is 9986115.
Edit: I also read that GM refuses to provide the exact specifics of “specification 9986115“ to the other fluid manufacturers probably because it doesn’t mean anything and they just want to corner the market on the oil and charge whatever they want.
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