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What is Gear Oil?

By Adam Hill
Updated Feb 16, 2024
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The vehicles we use every day, such as cars, trucks, and motorcycles, are complicated pieces of machinery, requiring proper maintenance and care. One product that is essential for the normal operation of these vehicles is gear oil. Gear oil is a lubricating motor oil made specifically for transmissions, differentials, and transfer cases in cars, trucks, and some industrial machinery. It has a high viscosity, or thickness, to ensure maximum protection of essential moving parts.

Gear oil is similar in some ways to the motor oil used for engine lubrication, but has many different properties in order to cope with the different demands that are placed on it. Motor oil, for example, needs to be able to take a great degree of heating without breaking down, since it lubricates the parts in an engine that are directly adjacent to where the combustion takes place. Gear oil, however, does not reach the temperatures of engine oil. In contrast, it is associated with parts that move somewhat slower and which are far away from the combustion in the engine, although it does get heated significantly as the vehicle is driven.

In places such as transfer cases and differentials especially, the pressures of the interlocking metal parts are much more extreme than similar pressures in the engine, and these pressures are what gear oil is made for. Some of the compounds in it contain sulfur, and the sulfur forms a protective coating on all of the metal parts it touches, preventing direct metal-to-metal contact. Without this coating, due to the extreme pressures which occur, these parts would heat up to the degree that it would be hazardous to the operating ability of the vehicle, and the parts would quickly wear out.

In most passenger vehicles, the gear oil is not something that needs to be regularly replaced, as is the case with motor oil. For motorcycles, on the other hand, it is recommended to change it every 3,800 miles (6,000 km). A motorcycle’s gear oil will deplete over time and lose its ability to protect the clutch and gears. Changing this type of oil in a motorcycle is similar to changing a car’s motor oil. It is simply a matter of draining the old oil, replacing the drain plug, pouring the required amount of oil into the transmission, and replacing the lid. In this case, it is important to use only oil that is made specially for motorcycles or scooters.

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Discussion Comments

By JimmyT — On Jul 10, 2011

I have never heard of sulfur being used as any sort of lubricant. How does this work? Does it just make a coating over the metal parts, or is it something more involved where the sulfur reacts with the metal to make it more resistant to wear and tear?

Also, do they make synthetic gear oil for cars and trucks? I know a lot of car enthusiasts argue about what type of motor oil they use in their engines. How does the quality of synthetic gear oil compare to natural oil?

By Emilski — On Jul 09, 2011

@kentuckycat - I found myself wondering the same thing. The article mentions that gear oil is used in the transmission. Since motorcycles are manually shifted, maybe it somehow wears out the lubricant quicker. I am not very familiar with how a transmission works, though, so maybe I'm way off base.

If my guess is right, it also begs the related question of: is there any difference between the gear oil used in automatic or manual cars and trucks?

By kentuckycat — On Jul 08, 2011

I never knew that there were different types of oil in an engine. I assumed motor oil took care of everything.

What is different about the inner workings of a motorcycle that the gear oil in it needs to be changed on a regular basis? Maybe another question is why a car doesn't need to have the oil changed. It seems like eventually the oil would break down.

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