What do the letter and numbers mean on motorcycle oil bottles (e.g. 10w40)?
As well as being defined by their type (e.g. Synthetic, Semi-Synthetic or Mineral), motorcycle engine oil is also defined by it’s thickness (or viscosity). The viscosity rating is normally displayed on the oil container by a series of 2 numbers separated by a letter (e.g. 10w40 or 20w50).
This grade is also known as the SAE grade of the oil (e.g. SAE 10w40). SAE stands for the Society of Automotive Engineers and they are one of the the bodies responsible for publishing the standards that govern Automotive Oils (2 other bodies that grade oils are API, American Petroleum Institute and JASO, Japanese Automotive Standards Organization).