Magnesium oil got its name because of its oily texture. Although it feels oily to the touch, in fact it is not an oil, but rather a saturated salt solution made of 69 percent water and 31 percent magnesium chloride.
The oily nature is caused by on the one hand the high concentration of the salt in the solution, on the other hand by the extreme hydrophilic bonding characteristics of magnesium chloride. Every molecule of magnesium chloride binds six water molecules. This hygroscopic property means the magnesium chloride stays permanently wet, which supports its absorption through the skin.
At these levels of concentration, no preservatives are necessary. Magnesium salt is itself a preservative and the salt concentration in a saturated magnesium chloride solution is so high that almost no germs can grow.