All
Quick 2c here:
- Auto transmissions usually have heat exchangers to the engine radiator (eg inside a header tank) for two reasons - faster warm up when cold (engine warms faster usually) and then to dump heat from the oil (cavitation heating of the oil in the torque convertor, friction). Any thermostat is usually just to stop the oil being over-cooled.
- Manual transmissions probably won't benefit from a heater siimply because of the oil viscosities ... most manuals specify a monoweight oil (80, 90) and if multiweight (80-90 or say 75-140) then it's going to get **thicker** as it heats, not thinner. Surely in that case you need cooling, not heating?
Maybe a manual might be specced for a monoweight oil by the OEM, and if so then going to the lowest for your running temperatures would be an advantage. My call would be to err on the side of caution - most gearboxes are 100% splash lubricated (some will have catch channels to gather spray & duct it into bearings too) so getting it wrong will lead to less average lubrication & so friction losses & wear.
HTH
Max
(old guy who's played with too many European cars with hot automatics and too many noisy motorcycle crash gearboxes
)