I think it would be really interesting to be able to do either long term testing or have some way to collect enough data to see if transmissions that get serviced last longer than ones that don't. Based on the small sample set on this forum, my bet would be that your odds are better by doing nothing other than possibly adding a cooler than by having the trans fluid changed. It seems that almost every transmission failure that I've read about on this forum is the result of a fluid change and there are very few or no transmission failures I've read about that were due to not changing the fluids or for any other reason in transmissions that had not been serviced. Based on very little data, I'd probably say that fluid changes done right really is the best choice, but unless you're doing it yourself and know exactly what you're doing, it's better to leave the OEM fluid in for life. And that there's a high probability that 300k miles or more is pretty consistently do-able without ever changing the fluid. So unless you're towing heavy regularly or working it really hard, I'd favor not changing the fluid ever.
Now for my hypocrisy...lol. I have a couple cases of WS fluid in my shop and was planning to change my 4Runner trans fluid at 75k before I sold it. I used it pretty hard and thought it was probably a good idea to exchange the fluid. I'll probably use the fluid to swap out my Tundra trans. Mostly because I have the fluid and the use history is unknown. Based on the condition of the truck and the fact that the inside of the hitch and hitch pin hole have almost no visible signs of wear or chipped paint, I'd bet a that the truck never once had a hitch put in or towed anything. I'll change that. Haha. First trip I'll take will be towing a travel trailer to Alaska in June. So it'll get a little workout. I've been watching the classifieds for a used space shuttle to tow, but the market right now is pretty tight.
I've been kicking around some ideas on how to make an idiot proof transmission fluid exchanger that would swap the exact amount of fluid back in as comes out. It would seem like that should make the exchange very reliable and avoid a lot of the under-fill issues we see. But I haven't come up with a way to do it inexpensively with easy to find off the shelf parts. The biggest issues I am struggling to find the solution to are how to make it automatic without electronics. The obvious solution I see is to use a vessel with a barrier so that the fluid pumped out displaces the same volume of fluid back into the transmission. The problem is that it needs to be a low friction diaphragm. So a floating piston type design won't work. And a bellows type separator is expensive. Not sure what else there is out there that would work well. I don't think air pressure between two sealed vessels is a good solution. And the second problem is temperature. The volume expands when warm so if you remove warm fluid and exchange for cold fluid, you'll likely end up with too much fluid or the reverse. So you'd need the new fluid to be at the same temp as the old fluid. And mineral based oils expand at different rates than synthetic oils. But the basic info suggests something like 10% expansion between 32* and 200*F. If the fluid going in is room temp and the fluid coming out is operating temp, you'd overfill by 8-10% using the same volume. That's over a quart too much. I'm guessing that's why the fluid exchange machines fail to get a proper fill.
So maybe it's just not practical to do a perfect swap without the final fluid check. IMO it would be pretty useful for Toyota/Aisin to re-design the trans fill system and oil pan so there's a wider range of acceptable fluid volume and ideally make it so that it can be checked when hot or cold. Maybe that means two check plugs in the pan and you use one or the other depending on trans temp. Having the trans pan just a bit larger so it could be plus or minus 1 quart would go a long way toward avoiding most of these issue I think.
I hope in this case the dealer figures out what went wrong and fixes it. The dealer should be following the FSM procedure to do the trans fluid exchange. I'm not sure why they'd do anything else. If you have a vehicle lift, the FSM procedure handy and all the tools, it's really pretty straight forward. And that's what you expect at a dealership - you pay more for them to do it by the book as intended by the engineers who designed it. Unfortunately that's not always what you get.
