Distilled or deionised water are equally suitable, though deionised is far cheaper. Rain water would do if you have a supply thats free of debris from roofs, etc. I find deionised is the most convenient for availability and cost.
The main reason to use these is because tap waters usually contain calcium and magnesium ions. These have to be sequestered out by the antifreeze to minimise scale formation. So less work for the antifreeze, which does far more for the engine than prevent the water from freezing.
TC
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