The viscosity grade of a lube oil is determined by the Society of Automotive Engineers (SAE). Oils can be separated into multigrade oils and monograde oils. Multigrade oils must fulfill two viscosity specifications, their viscosity grade consists of two numbers, e.g. 10W-40: 10W refers to the low-temperature viscosity ("Winter"), 40 refers to the high-temperature viscosity ("Summer"). Currently, most automotive engine oils are multigrade oils, while oils for restricted usage, e.g. for seasonally used engines like lawn mowers, are often monograde oils.

The SAE viscosity grades constitute a classification for engine lubricating oils in rheological terms only and are intended for use by engine manufacturers in determining the engine oil viscosity grades to be recommended for use in their engines and by oil marketers in formulating and labeling their products.

Two series of viscosity grades are defined in SAE J300: (a) those that contain the letter W and those that do not contain the letter W. Single-viscosity-grade oils (“single-grades”) with the letter W are defined by maximum low-temperature cranking and pumping viscosities and a minimum kinematic viscosity at 100°C. Single grades without the letter W are based on a set of minimum and maximum kinematic viscosities at 100°C and a minimum high-temperature/high-shear measured at 150°C and 1 million reciprocal seconds. Multiple-viscosity-grade oils (“multigrades”) are defined by all of the following criteria:

a. Maximum low-temperature cranking and pumping viscosities.

b. A kinematic viscosity at 100°C that falls within the prescribed range of one of the non-W grade classifications.

c. A minimum high-temperature/high-shear viscosity at 150°C and 1 million reciprocal seconds.

Low-TemperatureViscosities

• Cold-cranking viscosity at defined temperatures depending on the viscosity grade of the oil - determined according to ASTM D5293 (cold cranking simulator). The procedure simulates the cold start of an internal combustion engine.
• Borderline pumping temperature of max. 60 000  mPa.s at defined temperatures depending on the viscosity grade of the oil according to SAE J300. Determination of apparent viscosity and yield stress according to ASTM D4684, ASTM D3829, ASTM D6821 or ASTM D6896 (mini rotary viscometer).

High-Temperature Viscosities

    Kinematic viscosity at 40 °C and 100 °C - low-shear viscosity - determined according to ASTM D445, where the alternative method according to ASTM D7042 delivers comparable results. High-shear viscosity (10/s) at high temperature of 150 °C - HSHT viscosity - determined according to ASTM D4683, CEC L-36-A-90 (ASTM D 4741) or ASTM DS481.