I don't have any good reason. I do have an E4od with 246,000 miles on it and all I use is the factory reccommended transmission fluid and Lucas transmission treatement everytime I have it flushed and change the filter. It averages about every 8 months or so. I have maintained it like that since new and have no problems, knock wood. The E4od is a good transmission when you have the '94 or later with all the mods to make it last. It is expensive to rebuild and I'm just not gonna go synthetic. Your tranny, your choice, I just wouldn't tempt fate.