Originally Posted by SHAMUS
On another Toyota web site, a member showed broken trans parts from towing on the highway, in overdrive. The Toyota WS fluid simple broke down destroying the trans. It could not stand the high temps. I am taking my 2017 Tacoma to a trans shop who has a proper machine to completely remove the Toyota "World Standard" (WS) fluid; including the torque converter, cooling lines to, in, and from the radiator with a REAL high quality synthetic fluid. Yeah, it will cost some bucks, but I want a trans that will truly last over 100k safely, no matter the driving conditions. By the way, the poster was a mechanic with a shop, not just some guy.
I'm calling this questionable unless he actually knew the temps. For one, he should not have been towing in OD. It is well known that the driven gearsets take a beating in OD ratios and also create a ton of heat. And,which vehicle? The gen 1 tundras are known for having weak overdrives prone to shrapnel especially with towing. I owned a gen 1 with the stronger transmission, the 5-spd. That transmission was very well cooled and high temperatures simply were not a thing. If the A/C was off, it stayed around 165 during the summer, towing 4500 lbs, or 180 with the AC on. With 6,000 lbs, it pretty much stated at 180. It would exceed that on very steep climbs, we're talking the 25mph-30mph 4,000 rpm variety, where it would spike to 220-225F.
I can't really fault WS and heat loading here.