I was waiting to see if anyone replied to your questions. I'm not so good at engine issues, if that's what it ends up being.
I'm curious as to what kind of OP gauge you're running: Electrical, with a sending unit attached to your oil filter housing; or a mechanical gauge, with a direct oil line from the oil filter housing to the gauge?
NOTE: It only takes 8 lbs. of oil pressure to keep your OP light OFF. And that's pretty low. Also, the Blue Bible (Haynes Manual) states that you should not be alarmed if the OP light comes on at idle, and a normal idle can be as low as 650 rpms. But be VERY SCARED if the light comes on while driving at highway speeds!
My second electrical OP gauge (before the sender went out) read differently than the mechanical gauge I now have installed. My bet is that you are running an electrical gauge and that the sender is messing with you.
Also: My first and original OP gauge (electrical also) had something wrong with the needle. The needle would not go beyond, say, 20 lbs. or so, because something inside the gauge was getting in the way of its travel.
Good luck working it out. Sounds "electrically suspicious" to me! I hope I'm right!
David
Daily Driver:
1971 Series 2a, 88, Soft-top, LHD