As I understand, there are two ways to flush it: One way uses a machine to remove and replace all the transmission fluid, the other way only drains a portion of the transmission fluid and when you refill, you are mixing new with some of the old fluid. The first method is more expensive, but I use the second method at 25,000 mile intervals with Quaker State synthetic. If you use the first method, you should be able to go longer intervals between replacement. I have about 80,000 miles on the car with no transmission problems and the fluid looks clean.