Drop the oil pan and rig up a pump in a 5 gallon bucket of oil. Plumb a flex line to the oil pressure port instead of the gauge. Drive the auxiliary pump with a heavy duty 1/2" drill motor, and put a BIG drip pan under the engine. Look for excessive oil flow from rod, main, and camshaft bearings. That's how Dad discovered the cause of low hot idle oil pressure on the first few small block Chevy 265s he rebuilt in the late 1950s- - - - -worn cam bearings. They stopped doing "in the frame" piston ring and rod bearing rebuilds after that. Full engine overhauls or nothing at all from his shop!

" I plasti gauge all the mains and half of the rods. All within specs." "Within specs" sometimes isn't good enough. I shoot for the low end of the specification when possible. For instance a ".001-.003" spec bearing actually needs to be as close to the minimum if possible. One other thing- - - - -RUN- - - -do not walk away from anyone who recommends oil weights like 5W-20, 10W-30, etc. These engines were never intended to run water-thin oils. I buy 15W-40 oil by the 5 gallon pail and run it in everything from lawn mowers to Diesel engines in my tractors- - - -even my Harley Davidson EVO engine. Haven't had an oil related engine problem in the past 30-something years. "That's my story and I'm stickin' to it!"
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!