To the OP:

You use the term "flush" several times where "change" would likely be more appropriate. I don't know specifically about your transmission, my experience is with a series of Hondas with automatic transmissions. Typically, a transmission fluid change will change about 50% of the fluid, maybe a bit more. This is ok, and is what manufacturers recommend, at some service interval. Some service departments push flushes, but it's ok to stick to changing, at the schedule interval, or decrease the interval if you feel like it.

A transmission flush is not usually required. It's typically a 3 or more change procedure, with a short drive after each change, the object being to get as much of possible of the original fluid out. It's warranted if the fluid has become contaminated, say by accidental addition of motor oil to the transmission.

 

"Good for the life of the vehicle"

Remember that this means the guarantee period as far as Ford is concerned. There are two reasons to change oil, engine or transaxle, they are:

- Depletion of the additive package.
- Saturation of the oils ability to hold "ash" in suspension. "Ash" is the technical name for the microscopic wear products that accumulate in oils.

I wouldn't expect the transaxle to accumulate much, but there comes a time when, if one keeps a car for the "long haul" that it ought to be changed or chemistry analysis should be done to verify it is OK.

The "ash" in suspension will eventually increase wear rates and if you exceed the capacity to hold it in suspension the wear rates will increase greatly. This transaxle has no filter in it.

So what's one to do?

As I see it, and if you are going to keep it for the "long haul," I'd change the transaxle oil at some comfortable interval. For example I towed this summer and changed the oil on completion of the tow since that's a higher wear rate period. You might consider every 30k or 50k miles.

Just my thoughts.

PS The reason that a change is recommended after submerging it in water is that if water gets into the oil it forms acids and depletes the additive package very quickly... not to mention how poorly these bearings & gears work when they have to run on emulsified oil & water...

 

Flush vs drain & refill

On most automatic transmissions when you "change oil" you really only change part of the oil. For example on my Suburban you change 5 of 11 quarts in the whole system.

The "flush" idea is to hook up the oil system to a machine and run all of the oil thru a machine that finely filters all of the oil and exchanges most of it for new oil, at least that's the inference they want you to make.

On the FEH transaxle you drain all of the oil and refill it with new oil (or at least filter it and return it), so the flush term, as the industry would have you think of it, does not really apply to the FEH transaxle.

It is really like changing engine oil, you drain it all and replace it all.