Gear & axle oil: the forgotten oil
Everyone knows motor oil. But the gearbox and differential need the right oil too , and that is a different one. We explain the difference.
The typesWhy gear oil is not motor oil
In the gearbox extreme pressure acts between the tooth flanks. Gear oils therefore carry special high-pressure additives (EP additives) that hold a load-bearing lubricant film where motor oil would fail.
That is why gear oil has its own classification (SAE with different numbers than the engine) and its own API classes such as GL-4 or GL-5.
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The most important types
- Manual gearbox oil (MTF): usually API GL-4. GL-5 with more EP additives can even attack synchroniser rings here , so follow the exact spec.
- Axle / differential oil: often API GL-5, designed for high surface pressure in the hypoid gear.
- Automatic transmission (ATF): a completely separate, thin oil. Here the exact manufacturer specification is mandatory; the wrong ATF can ruin the automatic.
The rule of thumb from the engine applies here too: spec first, then the oil.
Said honestly
Many gearboxes are considered „maintenance-free“, that is filled for life. With high mileage or towing, a change is often worthwhile anyway, because gear oil ages too. „Universal“ gear oils that want to do everything should be treated with caution: GL-4 and GL-5 have good reasons to be separate. If in doubt, take the exact approval. We give you the facts, the decision stays with you.
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