engine oil gets hotter...
not the transmission oil... and it is relatively very thickk in viscosity..hence does not requires cooling...
and it wud not b feasible to cool the gearbox...
plus gearbox has different gears..which only tranform engine power depending on the gear ratio of the engaged gear... cruising on 150 kmph does not means ...that sumthing very rigourous is happening inside the gearbox...
inside the engine... burning..and extensive friction is taking place... thousand of time per minute...
i hope u got the answer