Hey Zollin, I have been using ZMAX in my 00 Impreza RS right after a 1000 mile break in period. It seemed to give the car a small increase in fuel mileage. I was using Mobil 1 but have since switched to Amsoil synthetic. As far as the long term affect on engine where from the product, I have no idea if it will help. The car already has 25k on it. If it helps the Mpg's alone over the life of the car I think it is worth it.
