Today I dropped 4 quarts of my transmission fluid and replaced with Castrol DEX/MERC "for older Fords" I have changed fluid a couple times before, but this time I noticed a huge difference when it shifted. I have a 2004 and the transmission requires Mercon fluid. It was way smoother when it shifted, especially at the lower gears. I have always used ford brand "mercon V / mercon." that says you can use it with both. But never felt such a difference with fluid changes. This stuff is awesome.