Don't even bother with magnesium oxide (the cheapest version that's most likely to be in your multivitamin pill).

That said, magnesium 'oil' is magnesium chloride and water (about 2 parts magnesium to 1 part water). It's called 'oil' because it feels oily.

If you've got a cramp, spray some on, rub it in - and if it works, you know you're deficient. You can do the same with magnesium sulphate (Epsom salts). Cheaper, doesn't feel as oily - and doesn't work as well. But it's cheap enough to dump a pound or so in a bath and soak.

Personally, I take the citrate as it's cheaper than as aspartate and as bioavailable, but BEWARE: it's also a laxative. Build your dosage up gradually.

Also (trivia) magnesium 'oil' makes a good deodorant. I figure 'why not?' as it's another way to get some Mg into my system