Just changing to synthetic oil doesn’t mean the oil lasts longer. It also has to be a long life oil with more additives to keep the contaminants from breaking down the oil. I find that it isn’t worth it unless you change the oil yourself. Some places charge $20 more per change for semisynthetic and then another $20 if you get full synthetic. There is no way full synthetic oil costs them $40 more than the regular mineral oil.