Wife has 2015 Jeep Grand Cherokee with 3.6L V6. About 140K on the odometer. Vehicle has had nothing but Oilzum 5W-20 conventional oil and Wix filters. Service performed as soon as vehicle tells wife oil change is due. Just bought similar vehicle for my use. Garage who serviced my new used GC said to use synthetic oil when servicing. Am I courting trouble if I use synthetic in wife's GC?