Considered to be an oil’s single most important physical property, viscosity is a lubricant’s internal resistance to flow at a given temperature in relation to time. Changes in viscosity can indicate improper servicing, contamination or lubricant breakdown.

Viscosity is measured at 40°C for industrial applications and at 100°C for engine oil applications. It is most commonly determined using a kinematic method with results reported in centistokes (cSt – 1 Centistoke (cSt) = 1 square millimeter per second). In addition to the viscosity result, the crankcase oil viscosity class of an engine lubricant may also be expressed as an SAE grade.

Viscosity classification systems provide a uniform and common basis for designating lubricants. The grade comparison chart below is a general guide that applies to viscosity at several different reference temperatures but does not evaluate total lubricant quality. These comparisons are based on a 95 VI product.

Comparative Viscosity Classifications