[Kube]

You're going to get as many differing opinions on what oil to use as there are belly buttons.
I was taught in school (GM) that most critical is to use what the manufacture recommends and stay with the identical brand throughout the life of the engine.
Obviously with today's chemicals and yesterday's (flathead) engines, the factory recommendation doesn't apply apples to apples.
I might suggest you "test" different viscosities and see what works best with your engine and driving demands.
I'd start with a non-synthetic 20/50 in your (used) engine.
Synthetic in an old (used) flathead is a waste of money.