Recommended oil viscosity for general riding and all temperatures is: SAE 10W-40.
We've also had success running diesel truck oil (Shell Rotella, Chevron Delo, Mobil Delvac) at 15W-40 viscosity. Diesel oil has the bonus of extra detergent additives as well as zinc which older engines need. Synthetic oils are are not recommended as they will cause your clutch to slip. We recommend adding 2-3oz. of Zinc additive with every oil change.