Magnesium oil generally is moisturizing because it is hygroscopic (moisture-absorbing). In a humid environment, it will especially draw moisture to skin. However, in dry environments it is less moisturizing. Furthermore, after it dries completely, minerals remain on the surface of the skin, and you may feel this residue as a dry, powdery, and even granular sensation. It is best to shower (or wipe off the residue using a wet washcloth) at least every few days because this mineral residue can gradually build up (layer upon layer), and thereby somewhat lessen your skin's absorption of fresh applications of magnesium oil.
I favor magnesium chloride flakes (try Ancient Minerals brand) over Epsom Salts (magnesium sulfate). Natural magnesium chloride is better absorbed and retained in the body than magnesium sulfate. I typically use magnesium oil daily and take baths (with Ancient Minerals flakes) once or twice a week. Adding Epsom Salts into a bath that already contains magnesium chloride flakes would not add any benefits, except maybe boosting total magnesium content. I would use Epsom Salts only if I did not have any magnesium chloride.
Epsom Salts work (and are readily available), but if you read the literature, including Dr. Mark Sircus's book Transdermal Magnesium Therapy (2007), you find that magnesium chloride is most effective for transdermal use. In my experience, direct application of magnesium oil is somewhat more cost effective per dose than Epsom Salt baths.
Before I discovered Ancient Minerals magnesium oil, flakes, and gel (which contains aloe vera), I used Epsom Salts with decent results. But there's simply no going back since I know the difference and have experienced it for myself.
I encourage you to experiment, and see what works best for you.