J.P.
Ok lets try to put this to rest. Would you run your engine with 20psi at 8,000 RPM? No way. Why not? Because at higher RPM's the load on the engine is higher. If the
oil pressure is not high enough to over come the added pressure put on the rods and crank to keep the bearings off the crank what happens to the engine? How about spun bearings thrown rods piston scrape and all kinds of other not so great things.
You need to have higher pressure to over come the high stress put on the engine at higher RPM's to over come the forces that are trying to force to
oil out of the way.
Glad I got that out of my system!
Ok now on to your question. First I have a few for you.
1. What weight
oil are you running. If the enigne was put together loose you will need to run a heavier oil to help over come the added space.
2. What oil pump are you running? If you run the enigne on the loose side you need to have a pump that can move enough oil to over come the want of the engine to push it out of the way. i.e. HV HP style pump.
3. Are you running an external oil cooler? If so what size lines are you running and are you running a stat in the system so it does not let the oil flow to quick and not cool?
4. Did you check the pick up to pan clearance to be sure it is not to close or to far away? To close and at high rpm's it will not get oil to it. To far and and it could be sucking air.
If you start it cold and it shows 55psi it sounds like the oil is to thin after it warms up. Try a SAE40 to see if it makes a change. 15 at idle is fine but it needs to get up to 10psi for every 1,000 you rev it to or you will damage it. Maybe not today but it will happen.