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SAE Designation of gear oils by viscosity
The Society of Automotive Engineers (SAE) established a viscosity grading system for gear and Engine oils.
According to the SAE viscosity grading system all oils are divided into two classes: monograde and multigrade:
Monograde gear oils
Monograde gear oils are designated by one number (70, 90, 140, 250, etc.). The number indicates a level of the oil viscosity at a particular temperature. The higher the grade number, the higher the oil viscosity.
Viscosity of gear oils designated with a number only without the letter “W” (SAE 80, SAE 90, SAE 140 etc.) was specified at the temperature 212°F (100°C). These gear oils are suitable for use at high ambient temperatures.
Viscosity of gear oils designated with a number followed by the letter “W” (SAE 70W, SAE 75W, SAE 80W etc.) was specified at the temperature 0°F (-18°C). The letter “W” means winter. These grades are used at low ambient temperatures.
Multigrade gear oils
Viscosity of gear oils may be stabilized by polymeric additives (viscosity index improvers). Viscosity of such gear oils is specified at both high and low temperature. These oils are called multigrades and they are designated by two numbers and the letter “W” (SAE 75W-90, SAE 80W-90, SAE 85W-140 etc.). The first number of the designation specify the oil viscosity at cold temperature, the second number specifies the oil viscosity at high temperature.
For example: SAE 85W-140 oil has a low temperature viscosity similar to that of SAE 85W, but it has a high temperature viscosity similar to that of SAE 140.
Multigrade gear oils are used in a wide temperature range.
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Designation of gear oils by performance
American Petroleum Institute (API) established a performance grading system for gear oils (mostly automotive gear oils). According to the system gear oils are designated by the letters GL (Gear Lubricant) followed by a number 1,2,3,4 or 5:
GL-1
GL-1 gear oil has rust and oxidation protection effect but it does not contain extra pressure (EP) additives. the oil is used in low load applications only.
GL-2
GL-2 gear oil contain more additives than GL-1, but without EP effect. It is used in medium loaded worm gears.
GL-3
GL-3 gear oil possesses light EP effect. It is used in non-hypoid gears.
GL-4
GL-4 gear oil possesses moderate EP effect. It is most widely used oil.
GL-5
GL-5 gear oil possesses high EP effect. It is used in hypoid and other highly loaded gears.
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ISO Designation of industrial gear oils
International Standardization Organization (ISO) established a viscosity grading (VG) system for industrial gear oils. According to the system industrial gear oils are designated by the letters ISO followed by a number equal to the oil viscosity measured in centistokes at 40°C (104°F):
ISO VG 32, ISO VG 46, ISO VG 68, ISO VG 100, ISO VG 150, ISO VG 220 etc.
