I recently conducted experiment on myself using Transdermal Magnesium Gel (not the "Oil").
My RBC level was 2.13nmol/L or 5.18mg/dL
I went through 2 bottles over a 2 month period, a 700mg/day dose. I allowed the gel to sit for 45 minutes to 1 hour.
My RBC level was 1.63nmol/L or 3.97mg/dL
I somehow went from mid-range to deficient. My diet, exercise, caloric content did not change.
Granted, the blood vials could have been mistakenly mixed up. But since (to my knowledge) there haven't been any comparative studies to see how transdermal compares to oral forms and placebos, this product remains suspect anyway. For those who haven't been using it, you can conduct a similar experiment for yourself.
I'm redoing the experiment using 800mg/day of magnesium citrate (better then oxide) for 45 days to see how that turns out.
If anyone else has tried something similar, please share. I'm curious of the results.