I don't personally recommend flushes, but that seems to be the popular thing. Ford's recommended service interval is 25k miles. However, that hasn't changed since newer fluids have been developed. You can run as long as 50k miles on the newer fluid if you desire, but when you reach the mark you should change the filter. A filter change replaces about half the fluid, or you can replace all the fluid using a fluid pump or disconnecting the lower line at the trans fluid cooler (radiator), and following the recommended procedure.
Typically, I don't replace AT fluid unless it smells burnt, is darker in color, smells like dead fish, or the vehicle has been overheated.
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Originally Posted by the user formerly known as ZX3_Chick
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