Gear oil distinctive smell

That strong, sulfurous smell from gear oil comes from special additives designed to protect metal under extreme pressure. These additives contain sulfur compounds that activate when gears mesh together with tremendous force, forming a protective film that prevents metal-to-metal contact. The same chemistry that creates the unpleasant odor is what allows your car's differential and transmission to survive the crushing forces between gear teeth. Without these smelly compounds, gears would quickly wear out and fail.
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