I don't know if Ford sells an additive either, that's why I was asking. The Mobil 1 synthetic is compatible with Mercon, and is easier to find tham Amsoil. When I do full fluid exchanges I change the ATF in the sump, then pull the return line from the cooler, start the engine and let 2 qts pump out, shut off the engine, replace the fluid and keep repeating until the fluid coming out looks clean. There isn't too much mixing of the old and new fluid this way, so maybe 1 or 2 extra qts. I figure 12 qts should do OK on this trans.
