My oil pressure gauge reads 20 psi at idle, then increases to 30 psi at 1000 rpm and then stays there all the way to 5000 rpm. Is that normal on a 1999 502 MPI?

with an electric gauge it very possible try hooking up a mech and check it

I agree, always double check with a mechanical gauge. I had false reading on my electric ones, but the mechanical showed it was fine. I replaced the electric senders and now they read right.

I have the same engine and after its last oil/filter change the pressure seemed to go down low also.I thought it was a bit unusual because nothing else was done and all materials used were the same as always.
I changed the filter again (only had about 2 hrs on the other one) and voila.. the pressure is back up to normal, or possibly better than it ever was !
Don't ask me why I'm just gald that for once the fix was cheap and easy !

Was it the standard merc filter both before and after?

NO.. Very limited choice of most things here in the triangle but I Like to go with a bigger filter if I can.. Had used A fram since initial run in oil change with no problems.Then this happened and I went back to the teeny Merc one and its OK..it seems !