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We replace the engine oil because the Viscosity Index Improvers have degraded and the oil no longer has the appropriate lubricating properties.

We replace transmission fluid for similar reasons, but far less often.

Motor oil is used as the transmission fluid in most motorcycles.

It seems like motor oil and transmission fluid are loosely interchangeable, in some applications. So the idea would be, why don't we use transmission fluid as engine oil and replace it less often? Obviously, if this really worked we'd be doing it. I'm curious about why it doesn't work.

Is there something about transmission fluid that makes it undesirable as an engine oil?

Or

Is there something about the engine environment that would mean we'd have to replace the transmission fluid just as often anyway?

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  • Don't motorcycle engines commonly work in entirely different regimes (far higher rpm, for one thing) compared to common car engines? That might have something to do with it. I'd guess it's at least a starting point. – a CVn Oct 21 '16 at 14:35
  • Yes, but not appreciably. RPMs are somewhat higher, depending on the bike (mine idles at 1k, cruises at 3k, and red-lines at 10k), but compression ratios and operating temps are also typically a little lower. "Different" yes, but I don't think "entirely different" is accurate. – Matt Oct 21 '16 at 15:07
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The engine oil has to deal with very high temperatures (best example: the piston rings). Also the engine oil degrades because it gets contaminated with soot, corrosive combustion byproducts and fuel to name the most important influences. Transmission fluid is not exposed to those.

In the realm of agricultural tractors there exists the so called "STOU"-oil, it can be used as engine oil, transmission fluid and hydraulic fluid. The oil circuits are separated between engine oil and others.

On some/many motorbikes there is one single oil circuit where both engine, gearbox and the clutch get feed with the same oil. Here it is important to use a specific motorbike engine oil, as friction modifiers (common in car engine oil) are highly damaging to the clutch and the gearbox needs a certain amount of high-pressure additives (not so common in car engine oil).

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  • So theoretically, even if transmission fluid had the right lubrication properties, the engine would still make it as dirty as motor oil. – Matt Oct 21 '16 at 15:09
  • Do they both get mixed up somewhere in the engine? I don't have a clue about transmission fluid BTW. – BraveNinja Oct 21 '16 at 17:21
  • @user3065750 no, they are separated. Only exception are some/many motorbikes where the engine oil also doubles as a clutch/gear fluid – Martin Oct 21 '16 at 23:11

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