Some pretty interesting statistics regarding oil temperature in various components. I have a 2016 Highlander LE Plus 3.5L V6 with the tow prep package that includes a "larger" radiator, transmission cooler, and wiring for trailer lights.
1. The transmission oil doesn't get above 150F at freeway cruising speed with ambient temperature at about 82F . I have a separate transmission oil cooler. Coolant temperature stays at around 185F while moving at 70mph.
2. I was driving up the mountains about 40 minutes at moderate speed and the transmission temperature got as high as 160F but stays around 155F for most of the drive up.
3. Engine oil and transmission oil are at around the same temperature.
4. Everything got really hot when I stopped at a rest stop, even with coolant fan running. Air flow is kind of important

I can use this as a baseline to compare with when I tow my little trailer on both the freeway and up hill in a few months. I thought I'd share this here in case someone may find the info useful. What are people using out there to monitor the towing vehicle?