JIMSS2F

Hello guys. I am Jim from Ohio and just registered for this forum. Haven't been inside of an engine for several years and found myself amazed at how far out of the loop I am on these new engines and all the changes that have been made. My unemployed neighbor called me saying that his 99 Gmc 5.3 suddenly lost oil pressure, so I have been scouring this site trying to get up to speed on the causes. Installed new (old stock) Gm pump, used the green oring. Got 50-60lbs pressure, GREAT!! FEW miles later no oil pressure engine clattering. Went back in to check oring, looked fine. Upon further study I found that because the pickup tube is 1st design (no groove) I should have used the black oring. Installed black oring. After a few miles at 50-60lbs oil pressure slowly dropped to 0. Let it set over night fired it up, got pressure up to 50lbs. Even though the oil looked clean on the stick (oil change had about 300 mi) I checked the filter and found a lot of thick black sludge in it. Changed filter, pressure ran a steady 60lbs at cold idle. 15 mi down the road pressure slowly drops to 0. Anybody have a clue on what is causing this? Sorry for the long post.