There seems to be a lot of controversy concerning the viscosity of the oil for any engine, especially for the classic Chevy engines. What are the clearances on all of the main, rod, and cam bearings? I have always felt (based on 50+years of building 216-235-261 engines, but with little scientific experimentation) that a SAE 30 oil has too high of a viscosity for a cold engine, but maybe not high enough for a hot engine. I am using Shell Rotella 15W-40 in my 278 cubic inch 261.
Watch where the "W" is in the oil viscosity description. It is almost always attached to the first number. The SAE 20-50W is probably 20W-50, indicating that the oil meets the SAE 20W viscosity specification at cold temperatures and the SAE 50 viscosity specification when hot.
I suspect that your pressure with the 20W-50 is higher than desirable, but that the pressure with the "straight" 30 is too low, at least when the engine is hot. Try 15W-40 or 20W-40.
