I recently stopped using Castrol GTX 5w20 in my '02 Ranger since the warranty is almost over at 35,000 miles. I tried Havoline 5w30 which is what I have been using in the wife's Cavalier and I have not lost any fuel economy. In fact, may last 2 tanks have produced 21 mpg and I have never gotten better than that. I can tell no difference in the performace based on the butt dyno. I will do a UOA after the second interval.