What type oil for a 91 Harley Davidson?

For a 1991 Harley-Davidson, it is recommended to use a high-quality 20W-50 motor oil specifically designed for motorcycles. Look for oil that meets or exceeds the API service classification of SM or higher and is formulated for air-cooled engines. Synthetic oil can also be a good option for improved protection and performance. Always check the owner's manual for specific recommendations and requirements.