I spent the last half hour researching this.... as one does
Bizarrely there's no definite answer.
Tap water
Softened water
Demineralised water
Deionised water
Distilled water
For each one you'll find experts that say that you should use it, and experts that say that you shouldn't (I have disregarded forum posts, of which there are many).
If I had to go by 'ask the audience', then I would use Deionised water because it has the highest ratio of approvers-to-disapprovers compared to all other types of water.
If I went by logical analysis then I would go with Softened water, because it seems like the best water for the cooling system. Though you can't buy it at your local petrol station of Halfords.
very true, it's very difficult to find an definitive answer regarding this stuff.
one thing that i have always heard from mechanics in the past was to use AC discharged water, it's probably in the softened water range, and probably the best of both worlds.