I replaced my cam and lifters, deleted AFM, had the ECM reflashed, and replaced the oil pump with a Melling. Got it all together ran great, no noise, solid and immediate oil pressure. BUT, during the first 1000 miles, I was getting an intermittent P0521, oil pressure out of range. There was no rhyme or reason, it happened once within 30 feet of leaving driveway on a cold start, other times 100 miles into a long drive. Gauge always showed good oil pressure for what it's worth(nothing). No noise, crankcase full, no other symptoms of low oil pressure.
It worried me, an oil pressure problem is not something I'll ignore. I racked my brain, trying to come up with a plan to diagnose it that did not include pulling the oil pan/diff again. The randomness of the failure is what puzzled me, not your usual low pressure at idle problem, it almost always occurred at speed. I finally decided to dump the conventional oil early and put some synthetic in there, something I planned to do after the first 1000 miles. So off to the zone I went. When I purchased the rig, there was a nice new Fram filter in the front seat which I happily used, my parts bill was getting out of control. But as I looked through the catalog, I saw the Delco filter listing contained a note ,PF46E contains a built in check valve. I opened up a Delco filter box, looks like a regular oil filter. I opened a Fram and an STP filter, both had a large rubber flap that looks like it was designed for anti-drainback. I bought the Delco anyway and guess what, problem solved. The girl behind the counter was trying to talk me out of the Delco filter because there was a special on oil/filter combos and the Delco was twice the price.
You can't put a price on peace of mind.