Lubricant Viscosity


Viscosity is a measure of a lubricant’s flow characteristics, or thickness, at certain temperatures. The low-temperature viscosity (the first number, 5W in a 5W-30 lubricant) indicates how quickly an engine will crank in winter and how well the oil will flow to lubricate critical engine parts at low temperatures. The lower the number the more easily the engine will start in cold weather.

The high-temperature viscosity (the second number, 30 on a 5W-30 lubricant) provides thickness, or body, for good lubrication at operating temperatures. A multi-grade lubricant (for example, SAE 5W30) provides good flow capability for cold weather but retains thickness for high-temperature lubrication.

Note installers should refer to the vehicle handbook to select the proper viscosity lubricant for the ambient temperature and operating conditions at which the equipment will be used.

Viscosity Index

Viscosity Index (VI) is an empirical number indicating the degree of change in viscosity of a lubricant within a given range. It is determined by measuring the kinematic viscosities of a lubricant at 40ºC and 100ºC and using the tables of formulas included in ASTM D 2270. A high VI signifies a relatively small change in viscosity with temperature, whereas a low VI reflects a large viscosity change with temperature. Most mineral base lubricants have a VI between 0 and 110, but the VI of synthetic lubricant often exceeds 120. The incorporation of polymers often increases the VI of the base oils to over 110.