Sorry, but caddilac 390 Question...

Maybe the pump is too close to the floor of the pan and not getting enuff oil. Sry I know you didnt want to pull the pan but the pump is almost certainly the problem. By the way did you get new motor for ur stang? Lates

Low oil pressure troubles make me think of a story that happened to my brother's friend. He had a well known, "respected" local engine shop build him a big block chevy and the build included a couple dyno pulls to verify the power it would make. It was a lot of money for the build.

Anyway the engine gets built the speed shop calls, says your engine is done and we have dynoed it, you're gonna love this engine. He goes to pick it up and and he's giventhe dyno sheet. Dynosheet said something like 580hp. Not huge a number, but this was back in like 1990. That was pretty good back then.

So he hauls this engine home and puts it in the car and hooks up everything including a mechanical oil pressure guage. After it's all hooked up he and another guy decide to fire it up and see how she's runs in a car. Hit the started and it's running, but no oil pressure, I mean ZERO oil pressure. They shut her down. Start checking everything. It all checks out. Only option left is pull it out of the car and inspect and/or fix the oil pump.

They put the engine on an engine stand, drained the oil and rolled her over to pull the oil pan. As soon as the oil pan was off they had found their problem. Their was a big piece of duct tape covering the entire oil pump pick up. The engine never saw the dyno and the "respected" shop screwed him over big time. They took the tape off. Buttoned it back up, filled it with oil and ran it on the engine stand and had perfect oil pressure.

Anyway, back to your post. If you have checked all the easy things, you're only choice is to pull the engine and ispect for internal causes. ie.. bad pump, blocked pickup, etc... I've heard of oil pump gaskets partially blocking the oil passage a few times.

are you sure the oil pressure sender is functioning properly? It sould be an inexpensive part. Go get one and try it out. That's the only advice I can give you...or get a mechanical pressure gauge.