It depends on the yr of the vehicle. Earlier models, pre 2002, had a dipstick and you could check fluid levels and top-up the fluid. Later model transmissions are a 'sealed unit' and according to Ford are designed to go 150,000 m without a fluid change. (You can tell that you have this type of transmission as it has no dipstick.). Some ford dealerships, I believe, will change the fluid at 30,000 m intervals, but I guess if you are willing to part with $180 they'll do it whether it is needed or not. The critical factor with these trannies seems to be the volume of fluid in the tranny. The fluid is pumped into the tranny through the drain plug with a special adapter tool. To get the amount right the tranny has to be at the right operating temperature due to the expansion of the fluid at higher temps. Dealerships and Tranny shops use a scanner to obtain the temp of the fluid so they can accurately guage the right amount of fluid. For the home mechanic this is a very difficult procedure and ill advised to attempt if there are no issues with the tranny. I attempted this and spent a day trying to get the level of fluid right. Without the right level the tranny would stick, crunch and thud. I eventually got it to work well enough for me to drive the explorer to a Tranny shop and was charged $70 for a flush, new filter and fluid. Also, note that the fluid needed is specifically Mercon V. At a local Auto parts place this was nearly $4 a qt. I ended up using 8 qts trying to get the right level of fluid. For $70 it's a no brainer. Leave it to a pro, not necessarily a dealer but a reputable shop that can do the job and not charge an arm and a leg.