Most of us agree that Toyota WS ATF should be changed no later than 100,000 miles under normal driving conditions. A few of us, myself included, prefer to drain and replace about 3.5 quarts rather than attempting to flush 14 quarts on a sealed transmission.

How often should one replace 3.5 quarts in order to maintain a mixture of low and high mileage fluid, such that average miles on the mixture does not ever exceed 100,000?

I created a spreadsheet to calculate it.

Change 3.5 quarts every 30,000 miles to maintain old-new mixture average fluid life under 100,000 miles. There will always be a small amount of the original WS fluid in the transmission, ~1 quart after 270,000 miles and ~1 pint after 350,000 miles.

I have these calculations in a spreadsheet. If you haven't changed fluid and want to know how to bring mixed fluid life down to a particular number of average miles, post your fluid change history and I'll calculate how many 3.5 quart changes are needed assuming a 14 quart total capacity.

I have a similar spreadsheet, works great! I do prefer shorter intervals to change it out though, personal preference.

I sold my 08 FJ with over 191k miles on it ask month. I only did transmission pan drain and fill every 10k miles. The first time I did it it was light brown with 15k miles. From that point I started my transmission service and after the second drain and fill the fluid always came out cleaner and cleaner. So my opinion I will doing the same transmission service as I did with FJ. BTW I only use Toyota WS transmission fluid.

How often did you do the filter?

I wouldn’t do the filter if you are doing regular drain and refills. I do mine every 15k and my transmission shifts amazing and the fluid is always nice and red when draining. I have 59k on it now and my trans temps are always around 130 in the winter and 167 in the summer.

Toyota says its non-synthetic WS is good for at least 100,000 normal duty miles, therefore I'm assuming a full synthetic fluid partial drain and refill should be good for up to 100,000 miles even if it's an old-new fluid mix, part WS and part fully synthetic (because lots of posts say the mix is compatible).

After reading the above replies, I think I'll keep a small sample of oil from each change for a visual comparison. I'll add an extra change if the sample fluid color change is not improving.

A 30,000 mile partial drain and refill gives me an average old-new mixed fluid life of 30000, 46875, 59531, 69023, 76143, 81482 and 85486 miles in the first 7 changes (210,000 miles). I'm allowing average mixed life to increase as the fraction of full synthetic oil in the mix increases.

Never! There is no filter in the pan it is just a screen just like the one in oil pickup in an engine.

Thanks, I had no idea.

I don’t actually think most people would agree with the first statement.
I do, but I think if your going thru the trouble to drain and fill, your more than halfway there to doing a full flush(using trucks pump. It’s only adding the steps of removing the cooler line and doing a fill check when your done, which a fill check would be good practice even when only doing a drain and fill.

I'm planning to measure the amount drained and replace with exactly the same amount to avoid having to measure ATF fluid temperature. Meanwhile, I intend to investigate using an app on my iPhone to read engine codes (including ATF temperature). If it works well, I may elect to revert to a flush and fill check per your advice. From what I've read, a flush changes 2 quarts at a time to avoid drawing air into the pump. Intermixing a fully synthetic fluid with some non-synthetic WX fluid seems likely even with a flush.

Is there a good how-to for the 3.5qt partial change?
Are you replacing washers every time? Got part numbers?
As far as my research goes, there is no aftermarket equivalent to Toyota WS ATF.
Yes, there are quite a few testimonials from folks who have used other ATF fluids including synthetics with no issue. M1, Valvoline MaxLife, etc.

This is an old BITOG thread for future reference.
Maxlife ATF for Toyota WS - Bob Is The Oil Guy

I swapped my 2013 taco ATF 3 times before I sold it with 75k, and everytime it was blacker than I thought it should be. I only replaced the filter and total replace once. I also only loaded heavy loads a few times totaling around 2000 miles, once a trailer. I used to take my motorycles back and forth to the west coast. I also live in a area that is up and down mesas and 7000ft. Anyhow; it doesn't hurt to change the fluids unless you wait until the rig has a lot of miles and its never been changed.

Not sure what you mean by "no aftermarket equivalent to Toyota WS."

Red Line shows their D6 ATF (GL-4) is a "suitable replacement for" Toyota/Lexus Type WS, among many other OEM offerings. I'm sure there are other suitable replacements as well.

I would not want to do a drain and fill that way myself. Sealed does not mean that a complete flush cannot be done. I owned a Volvo with a "sealed" transmission. Remember the fluid has to move through the system somehow.
I just had to find a place that had the correct fitting for that Volvo. Had to drive to an adjacent city to get it done, but it was worth it.

DIY maintenance quick reference - Toyota 4Runner Forum - Largest 4Runner Forum has 5 links discussing ATF fluid change and lists part numbers. I did not replace the overfill o-ring. I've not found a single post on this forum or other Toyota forums complaining about Valvoline MaxLife.

Valvoline Maxlife Drain-and-Fills (WS Compatability)
Superman3043's Build Thread - Page 39 - Toyota 4Runner Forum - Largest 4Runner Forum

I read an engineering research paper on transient fluid temperature response to automatic transmission shifts. Each time a shift occurs, a very small amount of fluid is subjected to high temperatures, which slowly results in accumulative fluid degradation over many miles. That's why I favor of a synthetic fluid.