We have an oil pressure gauge not working correctly. Changed the oil pressure sensor under distributer, changed oil etc.. has not pings or knocks. Starts out at 40ish psi then after about 15 minutes it continues to drop to almost 0..goes back up a tad under acceleration. Where can we hook up a manual gauge or is there another sensor just for the gauge itself. There is a plug type sensor with one wire attached near the oil filter and we unplugged that but still the same issue so not sure what that plug or sensor is for but we plugged it back in...any ideas?

If this is a high mileage engine with no rebuilds in its history, I will suspect high bearing clearance.
I found hot oil pressure should be about 5-10 psi at idle per spec. Further research concludes, cam bearings have a important roll in 350 oil pressure.
I suggest a full rebuild, either with oversized cam bearings or replace the cam also.

Has about 90,000 on a new crate small block 350

It'* weird your pressure starts at 40 cold, then bottoms out at hot. On my 2002 Vortec 350, the pressure is like 65 cold, and 20 hot. Another cause of low OP would be the pickup tube not sealing correctly in the pump. But this would cause foaming in the oil if that were the case. A relief valve could be getting hung up also, but personally, I find this hard to chew on because when I rebuilt my 350 in my S10, I put a new pump in it.

Do you remember what the OP was when you first put the engine in service?

Its always run between 40-50..I think we may hook up a manual gauge somewhere and see what it actually is.

That'* a good start.

Yeah, hook up a real gauge where the oil pressure sensor is now and see what happens.

My 1999 Vortec 5.7 has the same symptoms too. It started after intake manifold gaskets went bad and put coolant in the crankcase . . . which means likely excessive cam and main bearing clearance. It still ran fine for 50,000 miles with a main bearing knock every time I started it warm. Oil pressure came up as the engine speed increased and was showing 30PSI by about 1,800RPM.

If you haven't had regular oil changes it can cause the pressure sensors to fail. They get a small piece of debris and they will not read right. Let'* see what an actual gauge reads.

He did say he changed the OP sensor.