Well 50 I would say is slow in a McLaren but a Prado I would say that is moderate
Ah man, I need to commit this to a doc so I can copy pasta with the amount of people I have explained this to haha.
This will probably still be vague but better than nothing. (in this context 50%=trans operating temp 100%=redline)
Road speed on roads, it is more-or-less ok. If it is something around a ~15-20º grade on road and it is stop go/creeping traffic it will overheat within minutes if not less than a minute. I can watch it go from 50% to 75% within 10 yards or so— if traffic keeps on at that rate I have no choice but to pull over and let the trans cooldown. I guess I could go with Toyota's rec. & throw it in 4L in town.
Once temp is in the trans I have noticed that even when I am moving at road speed again, if that speed is less than ~30mph I can still watch the trans temp climb. Example, I nearly overheated 100% in Aug. driving up to Alta in Little Cottonwood Canyon, SLC). If anyone has been there/lives there you will know that the road in front of the Alta lodge is basically flat, but the temp was still increasing. What set the stage for the trans getting hot in LCC was construction around White Pine where they had to drop the 2-way road to 1-line with port-a-lights. There we were between 3-10mph as we crept forward.
I also went with the dealer foreman and drove around the avenues in DT SLC showing him how easy it was to put temp into the trans. That is when they advised driving in manual & 4L off road as much as possible.
Off road, pretty much as soon as I start going around 15 mph on grades ~5º+(?). Any terrain that I would typically use 4H for will overheat the transmission— so I do not really have much use for 4H simply because any situation that calls for it will overheat my trans. Below is the only way I have been able to keep the temp down while in 4H.
As for the 'spit' Toyota is having me rub on this wound (I wouldn't even call it dirt and definitely not a bandaid), driving in manual does help, but does not stop the temp increase—
it will help you though so do it! 4L will also help, but only to an extent. It just prolongs the temp increase out to ~1-2 hours before you will start to see the temp increase.
Off-road example, for the OP and anyone in AZ, I had to bail off the Tele track at Bartlett Lake due to the trans temp & time. From the start to Devils Hole in 4L (whole time) we had to stop to let the Prado cool down, we still had about 1/3 to go and decided we did not have time to stop a couple more times to let the trans cool off.
We decided to let 'er chill for a bit & then rip up the wash, which the wash is not steep at all, I am unsure of the wash's grade— guessing, I would say flat to less than 5º for like 97%. The terrain did not call for 4L and honestly, we did not want to drive that far on flat ground at 10mph—instead, to keep the trans temp down in 4H I had to go 30 mph through the wash, otherwise the trans started overheating. Temp stayed at about 75% for most of the drive out, but was increasing rapidly under 20mph.
These are only a couple examples, but I have redlined the trans temp a few times & run at like 80-90% often under an immense amount of stress & worry. Driving to and from AZ from SLC had me stress sweating the whole time waiting for the trans to give out. That drive made 1st & reverse incredibly harsh, someone looked at me with much confusion from the thud when I reversed into the gas stall.
Also, corporate and the dealer are aware of all of this & they know that I am worried about my safety and the safety of everyone around me. From the outside looking in, it makes me think that Toyota largely do not give a shit.
(This commentary is solely my personal viewpoint and is not affiliated with or endorsed by Toyota. Any experiences shared are based on my own observations, research, communications & general ownership experience. this commentary should not be taken as official Toyota statements.)