I have a 2005 4.3l Silverado which is just coming out of warranty. I have put almost 36K miles on the clock in slightly less than 13 months. During that time, the oil (Havoline 5W30) was changed religiously between 3000 and 5000 miles. I have used the maintenance doses of Auto-RX during each oil change (3-4 oz.) The OLM (which I hadn't planned on following) first went off at 18K miles - no kidding! This was 6 months after I bought the truck with 0 mileage, which gives you some idea of the amount of highway driving I was doing. By that time, I had 4 oil changes in the can (I was keen to stay on top of the changes simply for warranty purposes.) The OLM went off again at 27K miles. The truck currently has 35K miles on the clock. Now that the truck is 1K miles away from being out of warranty, I am planning on switching to the OLM for recommended change intervals. Does anyone think I should switch to a synthetic for these grossly-extended periods? I will continue to experience the same level and amount of highway miles for the forseable future, and I also realize that the OLM is calibrated for dino oil at the specifications listed in the owners manual, but would it make better sense to spend some extra dollars on a better-quality synthetic (like Amsoil or Pennzoil Platinum) and long-life oil filter, and do a periodic UOA? This would result on forgoing the use of Auto-RX, I would assume.