For the past 45k miles my truck has had its oil pressure drop to zero, within 20-30 seconds after start up. It was not the result of a bad sensor as I tried replacing that first, besides, the loud ticking that would almost always follow was a pretty good sign of the problem. If I immediately took off driving, the pressure would stay up, but I had to keep the rpm's up. Once the vehicle warmed up everything functioned normally until the engine got cool again. I bought an oil pump with the intention of changing it out but a friend told me to just change out the oil sump O-ring instead as usually that is the issue with no oil pressure in a 5.3. On a 2x4 Tahoe it is very straight forward to drop the oil pan to access the sump and access the O ring. You have to remove the cross brace under the oil pan which is 4 bolts. You also need to remove the two oil pan/bell housing cover bolts (they hold a small plastic shield in place that keeps road grime out the bell housing. You also need to remove two bolts that hold the wiring harness in place. A simple electrical connector on the low oil sensor then you can drop the pan by removing all of the pan bolts. Once the pan is down there are two nuts that hold the oil sump to the main bearing caps, as well as the small bolt that actually bolts the sump tube to the bottom of the oil pump. I removed the O ring and it was flattened out like a wedding ring, but not cracked or deteriorated. It was very loose when pulling out the tube as well. I put the new O ring on, which was $2 and shaped more like a fat doughnut. It was harder to press the oil sump tube back into place but it was fairly easy. Scotch bright pad was used to clean the grime from the block where the oil pan gasket seats. I had a bad oil leak forever and the pan gasket was the culprit as it had long since flattened out as well. Once the oil pan was cleaned up and both surfaces cleaned for a new gasket I had the truck back together and running in less than 3 hours. High oil pressure once again. With 526k miles this is the most extensive "tearing" into the drive train I have had to do, other than a 2-3 shift solenoid going back in the transmission a while back. Hopefully this thread will help anyone with similar oil pressure issues who thinks they need a new engine, when in reality you can fix it for $60 in parts and an oil change.