Ok I just dropped a 04 ls1 {22k miles}out of a gto in my silverado. I put a comp 220/224 (new) in it. I had oil pressure problem at first because i didnt replace the oring on the pick up tube. I have driven the truck about 25 miles and the oil pressure went from 40 to 30 lbs at idle. It sits at about 750 rpms at idle. If you bump it up to 1000rpm it climbs to about 38lbs. I have a nitrous tune from the old smaller cammed 5.3 in it. It still has the truck intake and 5.3 injectors. It has a bit of pinging under accel. { i think spark knock} I smelt the oil to make sure it isnt dumping too much fuel and maybe thinning the oil. It does have a fuel smell to it but not too bad. Any thoughts would help.

 

Are you saying it was at 40 psi engine idling cold, then dropped to 30 with engine idling after 25 miles of driving? If so, thats normal in my book.

 

If you are still running the 5.3 tune it will be way lean. Get that thing tuned ASAP. The knock you're hearing is probably from being too lean.