I wish I could find it, someone posted a video on here of a cam bearing, pan off looking up you could see the bearing had moved and was showing enough to uncover the oil passageway. that would be a major internal pressure leak.
so my 2 cents on oil pressure, atleast what makes sense to me. if you have zero pressure with a high volume pump after it's warmed up, that's a major leak, blockage or sensor failing in a less common way. for leaks there's a bypass valve in the pump that can get stuck and just dump all the oil back into the pan before pressurizing the engine. on some years there's what'd called a afm bypass valve on the inside drivers side of the pan. it's set at 60psi bypass, if it stuck open it could bypass a ton of oil.. as for the vlom gaskets, some preach they can leak, if you still have the afm lifters in place, it doesn't matter. they are vented when the vlom solenoids are not sending oil pressure down to the lifters. my disable kit I installed it has you literally cut the gaskets off those ports and leave them open to vent any backed up pressure that could unintentionally activate the afm lifter. I saw no drop in oil pressure cutting the gaskets off and leaving the holes open. if you have switched to non afm lifters, those ports should be blocked. but I find it hard to believe you'll drop to zero psi over all 8 being left open.
as for o'ring, instead of pulling everything apart to try new sizes, just find a steep ish hill and park nose down, the oil will cover the o'ring. if the oil pressure comes up, the o'ring is leaking, if not. eh. you can also add 2 extra qt of oil at idle. that will cover the o ring too. Just don't drive around much with 2 extra in there. I think there's 2 posts out of 509 where replaceing an o'ring fixed it completely.
mechanical oil pressure gauge is where to start, there's a oil port on a ls on the front drivers side of the block, it's in the oil galley right after the oil pump, before the filter, so that would rule out cheap filter lowering oil pressure.
if you do end up with the pan off again, look up in there and make sure all the cam bearings are where they should be.
at that point if you don't want to pull the engine to find the issue. might be interesting to throw one of the hybrid pumps on it for a science experiment. they fit a standard ls engine but are a vain style variable displacement pump more like what the newer LT engines have now, and will flow a very high volume of oil. total Band-aid but just for reference, the normal engine gm minimum spec for oil. pressure hot is something like 7psi at 1000 rpm. the hybrid minimum is 36psi hot at 1000 rpm. mine sets at 50-60psi at all times.
