Automatic fluid exchange

[R2D2]

I removed over half the fluid volume of the transmission by dropping the pan. You get a little less than half just by draining out via the drain plug.

And yes "dilution" is the entire point of the process. By doing so every 15k the fluid will stay very clean. I would take regular drain and fills any day over a flush once every 60 or 100k.

WS is no where near as "special" as Toyota is making it out to be, I can assure you. If it was major players like Redline, Amsoil, and Valvoline wouldn't be touting there fluids as compatible, appropriate replacements. I took the not mixing old and new fluid to mean top up with new fluid don't reuse the old fluid drained out. It really makes no sense why there would be incompatibility between used and new fluid other than the fact it wouldn't make sense to add back in fluid that may have been contaminated.

The engine oil comparison is irrelevant because the pan drain on the Yaris removes more than 20 percent of the fluid volume. Also transmission fluids don't have byproducts of combustion to deal with and operate in a "closed" system so contamination rates are much lower.