Transmission Fluid vs Gear Oil

Someone pouring transmission fluid in a car.

Transmission fluid versus gear oil — is there a difference? Automatic transmission fluid is a type of detergent oil that helps remove debris. It is also hydraulic oil that is a lot thinner than gear oil and flows much faster. Most standard transmissions though use gear oil, not automatic transmission fluid.

What Is the Difference?

Gear oil is very thick and can handle very high heat before it burns off and needs to be replaced. Automatic transmission fluid does get hot, and boils to the point that it expands and leaks out. Transmission fluid is a slippery liquid that acts as a lubricant for all of the moving parts inside the transmission. In an automatic transmission, this fluid also serves as a coolant and a viscous fluid that transmits power from the engine to the transmission.

Gear Oil

Gear oil is motor oil specifically made for standard transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of a higher viscosity to better protect the gears. The high viscosity or thickness ensures transfer of the lubricant throughout the gear train. This heavy oil can create viscous drag leading to inefficiencies in vehicle operation.

Transmission Fluid

Automatic transmissions use automatic transmission fluid. Manual transmissions use a variety of oils: Regular motor oil, heavyweight gear oil, or even automatic transmission fluid in some rare cases. Check the car’s owner's manual for the specific transmission fluid to use.

Knowing the difference between transmission fluid and gear oil could save your vehicle. Remember this the next time your in the automotive section!