Harley Davidson recommends using their own line of oils, specifically formulated for their motorcycles. The recommended oil for a Harley Davidson Sportster can vary depending on the specific model and engine size. However, a popular choice is the Harley Davidson Screamin' Eagle SYN3 Full Synthetic Motorcycle Oil.
It's important to note that using the correct viscosity and type of oil is crucial for optimal performance and longevity of your motorcycle's engine. It's best to consult the owner's manual or contact a Harley Davidson dealer to determine the exact oil specifications for your particular Sportster model. They can provide you with the most accurate and up-to-date information based on your specific motorcycle's requirements.