Gear oil is a fluid lubricant used in gears (gearboxes) for reduction of friction and wear of the gear tooth surfaces, removal of the heat generated by the operating gear and corrosion protection of the gear parts.  Gear oils should provide reliable, efficient (low friction), low maintenance operation of gears at different speeds, temperatures, oil contamination.  The use of good quality oil will extend the life of your Differential, Transfer Case and Manual Transmission.

The following properties of gear oils are important for their operation:

 

Types of gear oils
Combinations of additives impart special functions to gear oils:

 

Compounded oils are mineral base. They contain rust and oxidation inhibitors, demulsifiers and up to 10% of fatty oils for better lubricity. Compounded oils are used mainly in worm gears. The viscosity of compounded oils according to the ISO grading system is between 460 to 1000.

Extreme Pressure (EP) gear oils

EP oils may be either mineral or synthetic base. They contain EP additives, rust and oxidation inhibitors, anti-foaming agents and demulsifiers. The viscosity of EP oils according to the ISO grading system is between 68 to 1500.

Synthetic gear oils may be based on polyalphaolefins (PAO), esters oils or polyglycols. They may contain EP additives, rust and oxidation inhibitors, anti-foaming agents and demulsifiers. The viscosity of synthetic oils according to the ISO grading system is between 32 to 6800.
Synthetic gear oils are used for gears operating under extreme conditions: very low or very high temperatures, high pressures.

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.

 

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.

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: