Does using synthetic oil give better gas mileage and if it does, does it give enough better mileage to justify its higher price? I am thinking of using it in my 2013 minivan and I go about 9,000 miles between oil changes because I drive mostly highway driving. My minivan has one of those lights/messages that come on and tell me when its time to change the oil, so going that long between oil changes is still doing what the owners manual recommends.