[Kenth]

Use the correct viscousity for the running temperature of the engine.
The normal running temperature for most classic GM V8 engines is 195°F to 220°F and here is were the recommended 10w30 protects the engine.
Ram Air or S.D. engines with somewhat looser tolerances and no oil shields on valve springs require 10w40.
Mineral oil is fine for regular oilchange intervals, synthetic oil is optional for longer intervals.
For break in with a flat tappet cam use GM E.O.S. the first couple of oil changes.
After that oils with 1000-1200 ppm zink is just fine. Over 1800 ppm you can expect added wear and residue inside engine surfaces.

JMHO from 40+ years maintainance on my Pontiacs.