Calculating Viscosity Index Between VI = o and VI = 300

FOR OVER a quarter of a century the petroleum industry has become accustomed to quoting a single figure, called the Viscosity Index (VI), to indicate how rapidly the viscosity of lubricating and hydraulic oils changes with a variation in temperature. During this time the introduction of additives (in particular of VI improvers) has led to the manufacture of oils of such remarkably flat viscosity‐temperature curves that a procedure which initially was meant to be an interpolation between 0 and 100 VI (i.e. between steep and flat viscosity‐temperature curves, as for Gulf Coast and Pennsylvanian oils, respectively) is now also used for oils with VI values above 100, with the result that ambiguous values are obtained, particularly above 130 VI. The ambiguity arises from the fact that very high VI liquids can have the same index for two different oils whose viscosities are identical at 100°F. but differ greatly at 210°F.