for it being an old higher mileage engine that especially never has to pass emissions id run a diesel 15-40 assuming you're not gonna winter car it but rather summer drive it. the higher visc doesn't hurt a thing and shell t5 or t6 is stout for an old ohv. that engine was made in a time when oils weren't so skimpy and low saps. a diesel oil of today is more comparable to a gas oil of before. sure the base stocks are better but still.
