Now that I have 4500 miles and on second oil change, started to do some hard throttle runs up to 70 about once a week where it is safe, entering freeway, etc and I swear the mileage seems to be improving. Besides, the Magnaflow cat back sounds so darn good. Maybe these engines need a little "drive it like you stole it" once in a while to complete engine break in. Of course, all warmed up and miles since oil change low. I did regular Pennzoil (non synthetic) oil change along with Mobil 1 filter just to allow a little more break in miles before switching to Pennzoil Ultra Platinum full synthetic. I usually use Mobil 1 but am switching for the MS-6395 certification.