I've racked up 21k miles in 10 months and have done two oil changes. I used RP at 7.5k and 15k. When are you guys changing your oil 6 months or 15k? Since I changed my gears from the stock 3.42 to 4.30's I purchased a case of Amsoil gear oil and while I was at it bought 3 gallons of engine oil for the next change. Is it really ok to stretch out the oil changes to 6 months or 15k miles or is that just of you don't use the truck?