I just checked the Swanson site page for the Zechstein magnesium again. They use water for to get the magnesium chloride to the surface. So I think that being super-concerned about the quality of water to make your own magnesium oil is a red herring. I'll probably used bottled spring or distilled water to make my own magnesium oil.
Tepidarium, please post your experiences with the Swanson brand magnesium oil. I'm sure a lot of people here would like to hear about it.