I don't know if this could be the cause, but in the front of the engine is a 3 way fitting. One of the lines goes to the head and there is a restricted fitting with a very small hole in it. This fitting reduces how much oil will be pumped up to the rocker arms. I have read several times, that the oil pump is powerful enough to pull all the oil from the pan and dump it in the top end.
I replaced my original restricted fitting with a new fitting, filled it with solder and redrilled a hole to the correct size. I do not know if this would leave the bottom end dry enough to cause an issue, but it may be worth looking into.
Someone with more GMC engine experience should be able to confirm or deny this.
Sorry for your loss.
Jeff
