I have an '06 F-150 FX4 4x4. I flushed the trans at 30,000 miles with M1 Syn ATF. No problems for about the firt 1000 miles, then I started to notice a few things. From park to reverse pulling out of my driveway the engagement started to become rough. 1st to 2nd shift got to the point where I thought I'd bark the tires. Finally I was getting a howl on downshift and a shudder sensation. So yesterday I went and had it reflushed with Mercon along with some additive that makes it work like a MerconV. Anyway, now the trans is acting normal and it feels like I've gained about 40 horsepower. In retrospect I wish I had just pulled through and went to a place that has actual MerconV, but I suppose I'll just watch things and see how it reacts. So far it's like a new truck, and I'm not sure why the M1 ATF failed me.