I am not an oil expert but what i do know is ATF is designed as a hyd oil which will handle high temp but i am not sure it can handle the extreme pressure point loads that a high torque engine can exert on to the gear teeth. The oil has to protect the gear teeth contact point other wise the hard casing on the teeth will wear off, then you get noise and wine etc etc, This is what gear oil is designed for. Sure ATF will make gear changes easy but at the expence of the gear teeth over time i would think you would destroy a gear box eventually.