oil pressure?
 

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 !