1965 Pontiac's and most likely all other years, had an low oil light on the dash if they didn't have oil gauges. The sender would turn on the light around 3 or 4 psi of oil pressure, and it didn't matter which engine it was. Heavy oil just wears out distributor gears, put more strain on the timing chains or gear train, and in my opinion doesn't lubricate as good, its to heavy to splash around and get where it needs to be. Thicker oil can bring oil pressure up on worn out engines, but if it doesn't reach the top of the engine while cold, you wear the valve train waiting for it to warm up.
My 250 has 5-20 in it, runs 10 psi @ idle hot. My oil gauge is a stock 30 lb gauge, I don't know what the pressure is after 30 or care to know. Anything above not rattling the valve train is good enough!
I just rebuilt and fired up a 468 CI Pontiac, everything brand new in it. I used 30w Penn Grade break-in oil. I primed the engine with a Dewalt 1/2" high torque corded drill, the pressure gauge showed 60psi. It was all I could do to hold the drill, if you think heavy oil doesn't rob horse power, just prime one someday and see just what it takes to turn the oil pump.
