Honda recommends the oil viscosity for general riding and all temperatures as SAE 10W-40.
We prefer to use diesel truck mineral oil (Shell Rotella, Chevron Delo, Mobil Delvac) at 15w-40 viscosity. Along with a slew of detergents, there are additives like zinc that benefit older engines. We recommend adding 2-3oz. of Zinc additive with every oil change.
Should I run a newer style synthetic oil on my vintage Honda?
No, synthetic oils will cause the clutch to slip.