Gear oil is different from vehicle oil. Most people believe that SAE 90 gear oil is much thicker than SAE grade 40 or 50 automotive oil. In fact, their viscosity is the same. The difference lies in the classification used in the calculation. The former adopts SAE gear lubricating oil classification calculation, and the latter adopts SAE engine lubricating oil classification calculation. Another major difference is that they are produced with different additives.

Vehicle lubricating oil shall be able to resist the chemical by-products produced by gasoline or diesel combustion, and detergent, dispersant and other additives shall be added. Since the internal combustion engine is equipped with an oil pump and the bearings are lubricated with hydrodynamic lubrication film, the engine does not need extreme pressure additives used in gear lubricating oil.

Both engine oil and gear oil contain anti-wear additives, which need to lubricate, cool and protect components. The pressure of gear lubricating oil is high, which tends to form boundary lubrication. In this case, a complete liquid lubrication film will not appear between the two friction surfaces. For example, both car and truck differentials have ring and pinion hypoid gear sets. Hypoid gear sets can lead to boundary lubrication, pressure action and sliding action, thereby removing most of the lubricating oil from the gear. In order to resist the harsh environment, extreme pressure additives should be added to the lubricating oil. Top 1 lubricant adds extreme pressure additives to its gear lubricant to reduce wear and prolong the service life of gears and bearings.

Since many parts of the power train are made of ferrous materials, lubricating oil is required to prevent corrosion of ferrous parts and possible corrosion of other materials. Corrosion and corrosion problems (in the transmission system) are not as common as in the engine.

Many small and complex components of gear set in power transmission system will produce great noise and may bear impact load. The viscosity and extreme pressure formula of gear lubricating oil will reduce the noise of gear and dissipate the impact load.

Rotation of gear sets often results in foaming of lubricating oil. If the gear lubricants form a foam, because the suspended air in the lubricating oil is compressible, the load carrying capacity will be greatly reduced. For example, when the teeth are in contact with each other, the captured bubbles will be compressed, resulting in a thinning of the thickness of the isolation film. On the contrary, thinning the thickness of the isolation film will make the teeth directly contact the metal and accelerate wear. Gear lubricating oil needs to be able to disperse the entrained air to ensure the formation of a sufficiently thick lubricating film and prevent gear contact wear.