Although I agree that there are better fluids to use, ATF was factory fill and the recommended service fluid for nearly the entire production of the Contour. You didn't commit a great sin. Mobil 1 Mercon V will have a higher level of friction modifier that the original Mercon so additional friction modifier may not be needed.
My first choice would be Torco. Royal Purple has had good reports. Stay away from RedLine MTL.