IIRC, Magnesium Oxide can be useful because it's poorly absorbed, making it a better osmotic laxative than chelated forms / free magnesium. I agree it shouldn't be advertised as benefiting anything other than that though.
Chelated magnesium is definitely better, I've also felt benefits from free magnesium (you can buy drink mixes w/ MgCO3 + Citric acid that fizz off CO2 when mixed).
Chelated magnesium is definitely better, I've also felt benefits from free magnesium (you can buy drink mixes w/ MgCO3 + Citric acid that fizz off CO2 when mixed).