Half of your Mg is in your bones. Muscles have the next most, and then soft tissue. Mg is active in serum and is the 2nd major intracellular cation. Oral intake of Mg and its derivatives should replete your stores over time, but I would ensure adequate Boron intake to prevent Ca and Mg excretion and increase reabsorption.
A lot of the theoretical talk here is intriguing, but difficult to conclusively prove.
How does one apply Mg oil exactly? What is your skin prep routine? Is it Mg Chloride (Oil)?