I don't think synthetics are verboten in older engines. They just weren't readily available when stovebolts were new.
As far as synthetics staying on parts better than dino oil, that's not really a concern. The clearances and actual viscosity will keep oil in the bearings by capillary action. In fact a light viscosity synthetic will drain off quicker than a heavier dino oil. If you're worried about oil draining out of the bearings, throw some STP oil treatment in it. Actually I'm not sure that's even available anymore.
I'd go with 10W40 like SimS suggested. Synthetic is just too expensive to really be worth it.
