I have seen my 250 in middle of summer run 15 psi oil pressure with 5w20 oil. The stock oil gauge for '37 only goes to 30 psi, so I have no idea how much pressure it has above idle. I would't run 20-50 in anything, 10w40 is the thickest I would run.
If like Jerry already said, if the lifters and rocker arms are quite, you have plenty of oil pressure. I prefer more volume then pressure.
