assuming the coolant in the car is green, they can be mixed. BEST is to dump all the stuff in the car, flush it good and use the whole gallon plus water, 50/50. the mb coolant has no phosphates in it which react with aluminum, and there's lots in that engine. if for some reason the coolant is orange(not from rust), which i seriously doubt, it HAS to be dumped and flushed MANY times as that stuff and most normal coolant do not get along. remember, i'm assumng. i can't see your car, there light here sucks. good luck, chuck.