If you consider the original Harley oil, it has quite good features and keeps the engine at medium loads. AMSOIL oils are suitable for heavier loads and tuned engines. The main advantage of AMSOIL 20W-50 Synthetic V-Twin Motorcycle Oil is its viscosity, with the 16.3 viscosity index at 212F, which provides a high density of the substance when heated. Harley oil is not as good in this respect, thus, it's not that effective.               AMSOIL oil provides minimal engine wear, as it fully and evenly covers all the interior of the engine, giving maximum protection. The same applies to the transmission. Therefore, we can undoubtedly say that AMSOIL oil is much better than the original Harley oil in terms of viscosity and bike engine protection.