I just realized you're using pure magnesium oil, not nigari (magnesium chloride).
Nigari is the cheaper version of magnesium, it's used primarily to make tofu. When I first bought it from http://www.herbalhut.com
I ordered the Japanes nigari, which comes in flakes and also the Chinese nigari which comes in salts (similar to Epson salts) To make my own magnesium oil I just diluted (nigari) with water, 50/50. It does not itch at all.