The Limitations of the Viscosity Index and Proposals for Other Methods to Rate Viscosity-Temperature Behavior of Lubricating Oils 2012-01-1671
The Viscosity Index (VI), now defined by ASTM D 2270, is a relative number intended to represent the degree of change in viscosity versus temperature for lubricating oils. The basis for the rating scale, which was first defined in 1929, is a comparison of a candidate oil with two reference oils, one defined as “100 VI” and the other defined as “0 VI”. The VI scale has been widely used since its inception because of its simplicity and good correlation to a number of physical and chemical properties. However, the rating method suffers from a number of fundamental problems which are not realized by most users of the method today. This paper examines the assumption basis and historical development of the VI scale with an emphasis on the arbitrary and non-systematic manner in which the “100 VI” and “0 VI” reference series have been defined and modified over the years. This paper has 3 major objectives: (1) to educate users about the limitations of the VI method and the potential for misapplication and misinterpretation of an oil's VI rating; (2) to propose other, more systematic ways to define the viscosity-temperature behavior of a lubricating oil; (3) to stimulate further discussion and research on ways to improve the VI method.