Wow, I just got 16mpg with my 1999 4wd Ford extended cab 5.4L engine. Thats up from 10-11 mpg that I was getting this winter. I wonder if the warm temperatures helped me acheive this or if they started making the summer formula for the gasoline. I also switched to Amsoil synthetic motor oil from the Mobil 1 synthetic a few hundred miles ago. Maybe that has helped. The Amsoil does not seem to be leaking from the engine like the Mobil 1 syntetic did. I will keep an eye on the dipstick and see if this still holds true after a few thousand miles.