'Lo, Volks
Having read the excerpt from Prestone's pitch re the use of phosphate free coolant in Europe because of  "the high mineral content" and that "we do not have this type of water problem (in North America)". Lest we think that high mineral content does not exist in NA, it can and does (exist). Water is a great property in that it has the ability to hold and carry properties other than its own makeup of  hydrogen and oxygen. The simplist example is how it works with soap. Another is how moving water carries sediment. Anyway, water "takes on" whatever it can, if present and this includes minerals. I don't know anything about European water quality, but I do know that water quality in NA can vary greatly from location to location and that somtimes includes highly mineralized water. In other words, water quality can vary greatly and may or may not contain minerals to varying levels, even in North America. The point of this diatribe is to not assume that the water you use is low in minerals or free of them. IMHO, the use of distilled water in a wasserboxer is the way to go. If I go with the spendy VW coolant (and I do) then why not go with distilled water too?

Kit Carter, 85 Westy GL
Old certified water treatment operator