Gear oil is used to keep gears running smoothly. It protects the gears from wear down, corrosion, oxidation and rust. Gear oil is available in various weights. Applications differ by weight.
An oil's use can be determined by the SAE number or grade it is assigned. Numbers of 60 and higher indicate gear oil while lower numbers indicate engine oil. This means 75W-90 and 80W-90 are both gear oil.
The 75W-90 gear oil is used under extreme pressure conditions and is recommended for limited-slip differentials and for use in colder climates. Used in heavy duty manual transmissions, final drives and axles, high temperatures will not effect the heavy film left by this oil.
The 80W-90 gear oil can be used in extreme pressure conditions and as a lubricant for non-synchronised manual transmissions in heavy duty trucks or buses. This gear oil is recommended for conventional and limited slip differentials.