Early this year when I built my daily driven 360 I put a stock oil pump in it thinking everything would be all right.
Well, it wasn't.
Whenever the oil got warms up, the oilpressure at idle drops considerably to well below 20psi.
This was with 10w30 oil, to which I switched to 10w40 oil, which still has this issue.
I'm thinking the hydrualic lifters' oil bands are very close to the top of the lifter bores, causing them to bleed a lot more oil at low rpm and warm/thinner oil.
Short of changing the lifters (to solids) I plan to install a HV pump very soon first to see if this helps the situation.