viscosity index (V.I.) - empirical, unit less number indicating the effect of temperature change on the kinematic viscosity of an oil. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the V.I. of an oil, the lower its tendency to change viscosity with temperature. The V.I. of an oil - with known viscosity at 40°C and at 100°C - is determined by comparing the oil with two standard oils having an arbitrary V.I. of 0 and 100, respectively, and both having the same viscosity at 100°C as the test oil. The following formula is used, in accordance with test method ASTM D 2270:
V.I. = (L-U) / (L-H) x 100
where L is the viscosity at 40°C of the 0-V.I. oil, H is the viscosity at 40°C of the 100-V.I. oil, and U is the viscosity at 40°C of the test oil. There is an alternative calculation, also in ASTM D 2270, for oils with V.I.'s above 100. The V.I. of paraffinic oils is inherently high, but is low in naphthenic oils, and even lower in aromatic oils (often below 0). The V.I. of any petroleum oil can be increased by adding a viscosity index improver. High V.I. lubricants are needed wherever relatively constant viscosity is required at widely varying temperatures. In an automobile, for example, an engine oil must flow freely enough to permit cold starting, but must be viscous enough after warm-up to provide full lubrication (see multi-grade oil). Similarly, in an aircraft hydraulic system, which may be exposed to temperatures above 38°C at ground level and temperatures below -54°C at high altitudes, consistent hydraulic fluid performance requires a high viscosity index.