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