There are some hills where I live, so I regularly use the S mode and downshift to 3 or 2, so as to use engine compression braking instead of constantly applying the brakes. No huge NASCAR/Formula-1 downshifts.

I mentioned this at the desk of the shop I use for repairs, and the two guys at the desk expressed concern for the clutch pack in the transmission, premature wear caused by more shifting. But not concern for the engine. So, of course I searched this forum, found a couple of older posts, like:

Downshifting to use engin brake coming downhill

and

Transmission S shift issue?

Since our 5th Gen has been out 14 years or so, I figured if it was a real risk, someone would have experienced a problem with their transmission clutch possibly related to using the S mode and downshifting/compression braking.
Has anyone had that (bad) experience with the transmission clutch? Someone who drives in a mountainous region, perhaps?

I'm no expert, but based on the posts in that first, earlier thread, I figure that Toyota designed the transmission to stand up to this use, or why have the S mode? And I feel like I'm being clever by downshifting instead of riding the brakes, I hate to give that up. ;-) I've had several manual tranny vehicles when younger, enjoyed shifting, and I kind of miss that engagement with driving. Don't miss shifting when stuck in slow traffic, stop and go, etc., though.

This is a great place to get opinions, points of view, often very knowledgeable, usually from folks who know way more than I do. I am especially interested if anyone actually had to have a tranny repair, or hearing from mechanics who work on 4R trannys.

This should not be an issue when used properly. The trans in our 5th gens is an old school, well designed, transmission. The number of failures on this forum for the trans we have is a very small percentage.
THAT SAID keeping the transmission cool is the first law of keeping a transmission alive for the long run.

Personally I use my 2019 to pull a 4000lb Airstream basecamp trailer. I have been doing so for the last year and have had NO overheating issues with the engine or transmission.

I run a CFS dual core radiator, 4.88 gears w/ 285/75/17's, and a 11x8 inch Deraile trans cooler mounted in front of the radiator.
While pulling the loaded trailer engine temp runs 188-195, with the trans temp in the 155-170 range. I usually pull in 4th gear which puts the engine at +- 3200 RPM. I do shift to 5th on flat or downhill runs.
Worried about the transmission? No.

AMLOR

Downshifting is fine, especially when needed, like when towing. But if you are using it as your primary way of slowing down hill all the time you just traded some $40 break pads for a $5,000 transmission....

The extra heat and strain on the transmission is just going to put more wear and tear on it. But probably wont matter a whole lot with such a good transmission.

I have gone through more transmissions than cars, so I do lean on the side of caution.

It's fine, you'll never damage it by downshifting. The only real danger in downshifting is putting yourself into a gear that over drives the engine past redline, which this trans won't let you do.

The A750E and A750F

E= 2wd
F= 4wd

Has been used in 4Runners since 2003. It is a stout transmission. It has been used in many other applications too.

I live in a mountainous region, and will downshift as necessary. I'm not worried about the transmission.

We do a lot of mountain travel and I will continue to downshift until the 4Runners go all electric. Then we use the regenerative braking. Rented a Tesla and the regen braking is a cool feature.

When it's a hard downshift and I have time, I rev match it. The shift is so slow you can do it.

I think the only possible downside to downshifting below 4th for downhill grades is that the transmission only goes into lock-up in 4th and 5th gears. Which still really isn't a problem, its just going to generate more heat. Like others said, manage the heat and it's not an issue.

Of all the forums and FB pages, I've only seen like 2 people with transmission problems and both of those were due to an incorrect fluid flush and refill.

My advice: Just keep the vehicle in a lower gear, and Fck the people stuck behind you ...although use turn outs if you got more than 5 vehicles behind you.

Saves your brakes and your transmission.