Gear oil is supposed to STINK! It has an additive that makes it smell terrible. This smell does not come from use. There is no combustion process in a gear box to cause any build up of contaminants.

I change differential oil every 100,000 miles or so, and even this might be overkill.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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