I use (or, have used) all the oils described by Jerry, above (for the reasons Jerry nicely describes above).
Several years ago, my Suburban's 235 engine starting showing lower and lower oil pressure, until it was reading 0 psi at running temperature at idle (w/20W-50 oil). I took it out and put it to rest (and temporarily put in a spare 261 that I had). I did not want to throw a rod or hurt my 235 - I will rebuild it and put back in someday (with a later 848 head, a 261 cam, and a slightly larger carburetor).
Put in thicker oil and drive the engine. When the oil pressure gets dangerously low at idle, or when/if you hear an engine/rod knock, retire the engine.
