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Good points made thus far, as the temperature rises typically the viscosity falls, but if the temp gets too high it can damage the fluid or transmission. Just to throw out some numbers, Amsoil posts the viscosity of their fluids at multiple temperatures. For example:
Amsoil Synthetic Manual Transmission and Transaxle Gear Lube 75W-90 API GL-4 (MTG)
Kinematic Viscosity @ 100°C, cSt (ASTM D 445) 13.9
Kinematic Viscosity @ 40°C, cst (ASTM D 445) 87.6
Typically for constant load (cruising the highway) you will usually reach an equilibrium temperature, because the lower viscosity fluid will generate more heat until the temperature rises and the fluid becomes less viscous. This balances out with the heat being lost by the cooling of the transmission, either active or air flow.
Note this same principle also applies to other gear driven parts of a vehicle, such as axles, transfer cases, portal hubs, etcetera.
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