a couple months ago i bought a 1985 vw golf w/a 1.6l diesel.the odometer broke at 127k so i do not know the accutal millage. supposedly the original owner used only synthetic oil. there was quart of moble 1 15w50 in the trunk. with 127k plus on the car am i going to have to stick with synthetic or can i run none syn oil. if have to stay with syn do i have to stay with moble 1 and use 15w50 or 5w40 turbo diesel truck or can i use 5w40 rotella?