After watching the video posted above, which I have somehow managed to miss, and thinking more about this on a Friday as a means to avoid actual work, I'm may adjust my plan a little. The part of partial ATF replacement (that I was planning on doing) that I've never really liked, is that there doesn't seem to be any real engineering basis for how much old ATF is OK to leave in or even what "old" really is in terms of miles or the timing (miles) to do this. Fundamentally, this seems to be due to lack of guidance from Toyota. Different dealers do different things and hence all the threads and discussions.
I had been thinking of just draining the ATF from the pan which is about 4 QTs which is about 1/3 of the total AFT volume every 50K miles. Doing this effectively means that I'll be blending new fluid with an ever changing "old" blend. The table below shows what I mean:
View attachment 3531362
At 250K miles on the truck, I would still have 13% original ATF that has seen 250K miles. This might be fine or it may not be. Who knows. Why even replace the "lifetime" ATF at all? Well, "lifetime" to most of us is different than most car consumers. In my mind, there are only 3 reasons to replace the ATF. Either it degrades/breaks down with use or it gets contaminated from use with gear and clutch material debris or both. If we had fluid property and contaminate level criteria, we could just sample and test the ATF and do partial replacement at stay within the specs. To my knowledge we do not have that. In the absence of that, my guess is that 1/3 new fluid starting at 50K is probably good enough, especially if the bigger reason to replace is removing contaminates over fluid property breakdown.
What the video made me think about is that it really isn't too much more work to achieve replacing more than 1/3 the fluid by using the method in the video. Sure, its a little more effort but you achieve near total replacement with new ATF. So the next question is, is there any down side to replacing the ATF every 50K to 100K vs. 1/3 replacement every 50K. Its hard for me to see how running near 100% new fluid could be bad (not talking about a 250K tranny that's never been serviced). The only down side to me is the little extra effort and the cost of ATF. Though I have heard some people claim that there is some benefit to some level of contaminates as the transmission and ATF is broken in. I have a hard time believing that's true. Transmissions have ATF in them for only 3 reasons: lubrication, hydraulic and hydrodynamic forces to enable in shifting and torque conversion, and cooling. All of these things require a certain set of fluid properties that were determined by the Toyota engineers (or they built a tranny designed for the fluid they had).
So, my Friday afternoon conclusion here is that, for a vehicle that I intend to drive forever, replacing the ATF before it really needs to be replaced is better than waiting too long and my 1/3 replacement every 50K doesn't plan really have a solid basis and isn't really much different in terms of effort and cost (as DIY) than the video method. 50K miles is arbitrary and just based on discussions I seen and is almost certainly sooner than the vast majority if transmissions get serviced, even LCs. I think the time to do it is between 50K and 100K but that is just one guys opinion.