Modern transmissions spend very little time with the torque converter unlocked and don't have the extreme heat cycles as older transmissions. Ambient temperature has more effect on my transmission temperature than how hard it is pulling.
The ZF 8 speed my car uses the torque converter for in gear idle and launch only, it locks up as soon as the vehicle starts moving, basically turning it into a manual transmission that shifts automatically. It comes with a lifetime fill. Don't know if that is good or not but we will see how long it lasts, if I keep it that long.