273 rocker arm adjustment

Of course it has excellent oil pressure... you have 20W50 in there! Every pump will show higher oil pressure output into a given set of restrictions (i.e., an engine oiling system) with heavier oil as long as the pump can suck it up the tube, but this is because the flow rate is lower. The pressure sensor is at the start of the oiling path into the engine... it is a whole different story far down the oiling line once the oil gets through multiple restrictions. So pressure that you see on the gauge is not telling you the flow that you get up in the rockers.

Don't confuse pressure with flow.... 2 entirely different things.