Viscosity can be defined as the measured resistance to flow. The molecular weight and composition determine the viscosity. The Viscosity Index (VI) is a unit-less measure of the tendency of the lubrication to change viscosity due to a change in temperature. A low VI suggests that the lubrication will have a significant change in viscosity with a small change in temperature. Lubricants with high viscosities will create high levels of friction and therefore decrease seal life. Lubricants with low viscosities will reduce friction and power consumption. However, lower viscosities require an increased pump rate to maintain sealability (See Hydrodynamic Aids).

Parameters Affecting Sealing: