Flushes are a waste of time. Unless there has been some sort of damage to the transmission, or you've been hauling- the fluid should last more than 60k miles. I wouldn't even change the filter unless it was dirty. If it's still red, you're good to go.
Now if you've been overheating, then it needs to be changed.
Dexron fluids should not be used in a Ford. Most of what you're seeing on the shelf at parts stores is a fluid that will meet the specs of both Mercon and Dexron III. Mercon production ceased in 07. Mercon V can be used in vehicles that were spec'd for Mercon, but Dexron has different properties that will cause damage to transmissions using Mercon.
I am not sure if newer Dexrons are ok though that is true. I'd stick to the recommended fluid if the change interval has passed.
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Originally Posted by the user formerly known as ZX3_Chick
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