Oil gauge

I just bought a 95 transport and I noticed that the oil pressure gauge is acting funny. When I am going down the road the oil pressure is fine 40ish but when I stop at a stop sign or let it idle it drops down just before the red. I replaced the oil sending unit and now I am out of ideas.

It sounds to me that you have low oil presure at idling. What about if you rev the eninge in neutral. Does the oil pressure goes up to normal?

Have you changed the oil and filter. This would be the first thing to do.

Well...a low cost possibility: change the oil filter, and avoid FRAM. I had this kind of problem with FRAM filters, and once switched to DELCO, all returned to normal.

I just changed the oil and I used a delco oil filter and I am getting the same result. In neutral when I rev the motor it does go up.

Well....this may indicate slightly degraded oil pump or worn bearings.....but not a really severe issue.
whatever the case, as long as you have adequate pressure under load at rpm higher than idle, there is no panic.

As the saying goes about oil pressure, "..any is plenty"....(while I do not totally agree with this: I do like more than less)....but it does ease the urge about that issue.

YOu did not mention negine type....3800 pump is front of engine, behind balancer pulley, while 3.1 is a classic cam driven pump, accessible from the oil pan...but I'd live with it for a while before going to surgery....if ever...
IN fact...my '93 3.8 pressure also goes down at idle....but I did not pay enough attention to recall to what extent...and it runs near to 40 psi or a little bit below normally, but not higher than that unless cold.

The engine is the 3800. How hard is it to change the oil pump?

The engine is the 3800. How hard is it to change the oil pump?
I know this is an older thread - but what oil? Changing from 5W20 to 10W40 or 15W40 makes a BIG difference in idle oil pressure.