I plan to change the coolant on my car soon, the local BMW parts department told me over the phone that they only supply the coolant ready mixed in 1 litre bottles, but I'm guessing they're mistaken and it's concentrated. Has anyone either changed the coolant or got some to top up with and can tell me the correct part number and if it is concentrated or pre-mixed to save me the hassle of trying to get the correct information out of BMW?