Viscosity Index

Viscosity index ( \(VI\) ) is a measure of a fluid's sensitivity to change in viscosity with change in temperature.  The lower the index, the greater the change of viscosity of the oil with temperature and vice versa. It is used to characterize viscosity changes with relation to temperature in lubricating oil.  The viscosity of liquids decreases as temperature increases.  The viscosity of a lubricant is closely related to its ability to reduce friction.  Generally, the least viscous lubricant which still forces the two moving surfaces apart is desired.  If the lubricant is too viscous, it will require a large amount of energy to move.  If it is too thin, the surfaces will come in contact and friction will increase.

Viscosity Index Standards

ASTM Standards

• ASTM D2116 - Standard Method for Conversion of Kinematic Viscosity to Saybolt Universal Viscosity or to Saybolt Furol Viscosity
• ASTM D2270 - Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C
• ASTM STP168 - ASTM Viscosity Index Calculated from Kinematic Viscosity
• ASTM STP532 - Viscosity Testing of Asphalt and Experience with Viscosity Graded Specifications