The back of the Audi G13 coolant concentrate bottle I have specifies "use with distilled water ONLY", is it OK to use deionized water instead, or any other kind of water?
I’ve read a great deal of opinions on this topic but am looking for the facts, opinions on this matter vary wildly from "it doesn't matter" through to "you should only ever use [distilled or deionized] water" (and opinion varies on which is more pure), some even say that tap water is preferable!
Distilled water is quite scarce in the UK at least, instead everywhere will sell you deionized water and will treat both as one and the same, when in fact according to Wikipedia they are different.
Things are made more confusing when you look online to purchase distilled water and discover that manufacturers boast that it has also been through reverse osmosis and deionization!