Back in 1993 when the American Petroleum Institute first introduced base oil groups, it provided a useful way to sort base oil types by performance characteristics. In addition, it afforded an opportunity for automotive engine oil blenders to make marketing claims by using certain base oil types in their finished products. But as helpful as it was all those years ago, times have changed and questions are mounting about the utility of the API Groups and whether change is needed. Understanding why starts with an overview of the current system of classifying base oils.
The API categorizes base oils into five Groups: I, II, III, IV and V. Each is defined by a limited number of physical and chemical properties, including sulfur content, saturates level and viscosity index, which loosely correspond to how they are manufactured.