I have heard that the trucks using the def fluid get a lot better milage than the regular egr in the previous generation of diesels. I have a friend that that went from 6.4 powerstroke to 6.7 and is much happier with his fuel mileage. But it also cost him a higher premium. I know he was upset that his brother had a v10 getting the same mileage as his 6.4. Anyone with any knowledge on if def is helping?