Depends how old your car is. I was told that the washer jets that produce the fan shaped output on the windscreen are a lot finer than usual and that mixing different washer fliuds can cause the fliud to sort of gel up (to be fair I saw this on a Ford) and block the washer jets. According to the dealer if you don't use VW washer fluid and the jets do block that's not included in the warranty, if you do then it is. Don't know how true this is of course!