The recommended engine oil viscosity for a 1980 Honda CB 650 is 10W-40. This viscosity provides good protection for the engine components while maintaining proper oil flow. It is formulated to minimize friction and wear while providing adequate cooling for the engine.
In general, Honda recommends using an SAE 10W-40 or 20W-50 oil for temperate climate conditions. However, it's essential to refer to the specific owner's manual of the 1980 Honda CB 650 for precise oil viscosity recommendations, as certain factors such as geographic location, climate, and personal riding habits may also influence oil viscosity selection.
