Recommended oil viscosity for general riding and all temperatures is: SAE 10W-40. We recommend adding 2oz. of Zinc ZDDP additive per quart of oil with every oil change.
We've also had success running diesel truck oil at 15W-40 viscosity. Diesel oil has the bonus of extra detergent additives. Synthetic oils are not recommended as they will cause your clutch to slip. As of late 2023 Shell Rotella T4 is now a semi-synthetic oil and causes clutch slippage in the engines. Chevron Delo 400 and Mobil Delvac 1300 are still mineral oil.