Gear oil is a lubricant used specifically for automotive manual transmissions, differentials, transaxles, and transfer cases. It helps the drivetrain run smoothly and protects critical internal parts in a vehicle’s gear systems from wear and heat damage. Inadequate lubrication can result in scouring, corrosion, scuffing, and pitting of important drivetrain components. This deterioration can compromise a vehicle’s performance and lead to costly repairs.
Gear oil has a higher viscosity than engine oil. It is identified by its performance classification and its viscosity. The Society of Automotive Engineers (SAE) publishes a viscosity grading guide (J306) for gear oil, which is a different grading scale than the SAE’s engine oil scale (J300).
Gear oil API performance classifications
There are several automotive gear oil designations, but the two most common gear oil designations – Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles – are set forth by the American Petroleum Institute (API) for passenger vehicles:
API GL-4 classifies passenger car and light truck manual transmission lubricants and oils used in tractor gearboxes and spiral bevel gear final drives.
API GL-5 defines passenger cars, light, and heavy trucks and buses as hypoid final drives with a large amount of sliding movement across the gears.
A number of automotive manufacturers require lubricants that meet their own in-house specifications. Mercedes-Benz, GM, Ford, Volvo, and others have their own specifications for the gear lubricants to be used in their equipment, particularly where special warranty or extended lubricant service is offered.
Gear oil viscosity
There are two common passenger vehicle gear oil viscosities.
75W-90
The 75W-90 gear oil is used under extreme pressure conditions and can be recommended for limited-slip differentials and for use in colder climates. It is commonly used rear differentials, some final drives, and axles. The thick oil film formed by this oil can withstand high temperatures.
80W-90
The 80W-90 gear oil can be used in extreme pressure conditions and as a lubricant for some non-synchronized manual transmissions in heavy-duty trucks or buses.
Monograde Gear Oil
The SAE (Society For Automotive Engineers) classifies gear oils suitable for operation at one temperature on a monograde scale. Gear oil meant for high ambient temperatures is designated by a single number, such as SAE 50. Higher numbers indicate oils with higher viscosity.
Multigrade Gear Oil
Multigrade gear oils have additives that change their viscosity at different operating temperatures. These gear oils are labelled with a rating for the oil’s low- and high-temperature viscosity. For example, SAE 80W-90 gear oil has a low-temperature rating of 80 and a high-temperature rating of 90. They perform better than monograde oils.
GEAR OIL EP 100 
