Methods for Rating the Viscosity-Temperature Behavior of Polymer-Containing Lubricants

The Viscosity Index (VI, ASTM D2270) has a long and widespread history as a rating scale for the viscosity-temperature behavior of lubricants. However, the method has serious technical deficiencies which can lead to misleading conclusions. We have examined alternatives to the VI scale in an effort to more accurately characterize the viscometric behavior of modern lubricants, which are formulated with highly refined base oils and advanced polymers. We have found that the VI method can actually mask improved viscosity-temperature behavior for certain highly additized polymer-containing lubricants. We examine two different methods, the Roelands Slope Index and the Proportional VI, as an alternative to VI. It is important for lubricant developers and consumers to be aware of the limitations of the VI scale in describing the viscometric qualities of modern, high performance lubricants.