UPDATE to an older but relevant thread - We just hit 60k so I felt it was time to have the transmission serviced based on some advice received here. There is no way I'm waiting until the recommended 100k+.
We do tow from time to time and also have a lot of stop and go traffic (Atlanta), for what that's worth. In my opinion, the dealer prices are absurd and our local dealer is famous for their terrible service department, if you can even reach someone there at all.
I phoned a few places and when I called Goodyear, the guy immediately said, "we have to drop the pan to perform the service, and we can only remove about 80% of the fluid". They obviously do a lot of these because he was familiar with the vehicle, even noting that there wasn't a dipstick to check except the one underneath, near the pan itself.
Anyway, it was about $200 and they added / changed 10 quarts (per the receipt), and replaced the filter and gasket. The fluid they recommended was Valvoline Maxlife ATF full synthetic. If that's the price every 60k, I can deal with that.