I've used Mobil 1 in a couple of cars and highly recommend it, but I think it would be awfully hard to measure any mileage difference in the real world, even harder in something like a tractor. I just bought a motorhome with a 330HP Cat that I am going to switch to synthetic this weekend. Any help with 7.5 MPG would be nice, but I think the synthetics mainly help reduce engine wear.
I had a 328 BMW on Mobil 1 that I ran the wheels off of up and down the mountains in Southern California /forums/images/graemlins/smile.gif. When we had is smogged, they said it was the cleanest engine they had ever tested.