Not only will it overflow if you add washer fluid while the system already has washer fluid in it but the owner's handbook recommends to add washer fluid when you notice that it's empty. Do you have an owner's handbook? It's a very good investment if you don't; i.e. what weight of synthetic motor oil did you put in? Chrysler/Mercedes recommends 0w-40 or 5w-40 full synthetic; not 5w-30 or 10w-30. I have a toll free number at work where you can order a handbook & have it sent to your address. I'll get the number tomorrow & then post it then. The toll free number for ordering an owner's handbook from Chrysler is (800)890-4038. They will send it to your address with no middle man (read, "dealer") involved!