7000# trailer across PA and back in 90+ degree heat the highest trans temp was 223 (had display on entire time and temp never above 218 on highway - cruise on between 60-75 mostly). 95+ degree heat at lower country road speeds I saw 227 after a slow crawl/stop and go uphill to a stop sign. It dropped right back down once underway again.
I'll be keeping an eye on my temps even closer now when towing! 240 is (used to be) cooking the trans fluid.
I read somewhere that this 10 speed is filled for life.
Not sure what temp Mercon ULV starts to break down, but, I know traditional trans fluid broke down exponentially with temps.
In my '05 tacoma, hottest probe was at the torque converter (with factory trans cooler). It'd go to 235 for a little bit and then bump down depending on how hard it was working. Pan temp never over 225. But this alone made me replace trans fluid. It kept going no problems ~1000 miles towing at max rated capacity...
This is what I read:
8. Transmission Fluid
Mercon ULV, a transmission fluid specially designed for the 10-speed transmission, follows the trend of ever-thinning viscosity experienced by transmission fluids and engine oils across the board. Considering that the viscosity of most transmission fluids fall between 6-7.5 centistokes, Mercon ULV’s 4.5 centistoke rating is impressive. Reducing both friction and risk of hydraulic pump overload, the fluid (combined with a new generation of filter) plays a key role in regulating temperature and efficiency. It is also designed to last the life of the transmission
https://www.autoinfluence.com/11-things-you-need-to-know-about-ford-gms-10-speed-transmission/