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This is a measure of a fluid's resistance to flow. It is ordinarily expressed in terms of the time required for a standard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since, viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is now commonly reported in Centistokes (Cst), measured at either 40°C or 100 °C (ASTM Method D445 - Kinematic Viscosity). An earlier method for reporting viscosity in North America was in Saybolt Seconds Universal - SSF (ASTM Method D88). Other less common viscosity units are the Engler and Redwood scales, principally in Europe. (See also BROOKFIELD VISCOSITY, POISE).

The measure of the rate of change of viscosity with temperature. This change is common to all fluids - some more, some less. heating tends to make them thinner - cooling thicker. The higher the V.I., the less the tendency for the viscosity to change. V.I. is determined by formula from the viscosities at 40°C and 100°C in accordance with the ASTM Test Method D567 or D2270. The latter test is required for V.I.'s above 100. High V.I. oils are often preferred for service in which a relatively constant viscosity is desired under conditions of varying temperature. Some hydraulic systems require this property. Paraffinic oils are inherently high in V.I. and the V.I. of any petroleum oil can be increased by the addition of a V.I. improper. Naphthenic oils are inherently low in V.I. and aromatic oils are still lower - often having negative numbers