I'm not sure of the torque ratings of the 4 speed trans, I know the
3 speed is rated at about 250 ft lbs, but the 4 speed is a Toyota corp
made trans, and I have not heard of power related issues, most issues
are service (or lack of) issues but total failures are not common.
A trans cooler is a good idea, as is a high quality ATF and regular service
at about every 50K miles, so adding a drain plug and a spin on filter would
be a smart idea, most people don't change the ATF because it's simply a
pain to do, ATF in your arm pits etc
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Wild