Originally Posted by Adam65
I can't believe nobody has mentioned the HV pump. You are pumping all that oil right up to the top of the engine and it probably has had no drainback work done in the heads or the lifter valley. The fact that it's 20w50 only makes it worse-just returns to the pan that much slower. Under 3500 rpms may be the sweet spot where it just returns fast enough to be alright. Over that, and it doesn't come back to the pan fast enough.
As far as any pressure issues, they're fine up to when the problems start. If your clearances were fine on a stock motor, I would run 5w30 or 10w30. Just a rering and good clearances wouldn't dictate thicker oil. But verify that you have a good gauge as mentioned and a good filter (like Wix)-not Fram. Not sure about the delco, but the smaller one is probably hurting you here.
Did you verify the pickup to the pan clearance? If it's too low or too tight to the sloped up part of the pan sump, the pump will cavitate and not be able to pump oil at a certain rpm. In that case, though, the pressure usually comes back as soon as you let off the throttle.
Just to clear up a few things and possibly unearth whole new questions,
#1 I was running 10w30 at original startup and break in. Had the same problem as with the 20w50. I went with the valvoline 20w50 for the zddp for the flat tappets.
#2 even if we stop and swim for 15 min the psi still acts funny (which would make me think its not the oil not returning to the pan). Only after sitting overnight does it run in normal range again. Also, its a ski boat and there is a pretty big angle at which the engine sits. Dont think oil can sit up in the heads so much, it all runs to the back of the head pretty quickly (found this out while adjusting the valves with cut off V.C.'s). Also after returning to over 2000 rpm's the pressure is right back up to 40 psi which would indicate there is plenty of oil in the pan, only after dropping to idle does it drop out to nearly nothing.
I am in no way "trying" to discredit what you are saying and I appreciate the feedback, but these are the reasons Im not certain if what you are thinking may be exactly right.