LOL we replaced the oil pump with a HV, pan gasket that was leaking, the crank bearings since we were there, and I switched to 20W50 dyno oil 12 years ago and 70,000 miles ago on my 87 Renix and it is still one of my DDs.
I get 55 psi cold, and 22 psi hot at idle after a hard high rpm run in 100 F heat waves.
Only other problem I had was a slow gasoline dilution in the oil dropping the pressure over time as the leaking valve seal on #1 and #2 fouled the spark plugs slowly....New valve seals fixed that for a while, now 12 years later I am going to get new valve stem guides...or swap the head with my spare. Have you considered checking for gas build up in the oil? Oil analysis?