MAgnesium Citrate is pretty much like oxide but with citric acid , the reason behind is in order for the magnesium to be absorved there has to be enough acid in the stomack therefore the citric acid help to that purpose. As a frendly advise you should avoid eating grains like whole wheat, as they have phytates than inhibed the absortion of this mineral and other important one us zinc etc.
Change for a while to white bread and your body will start absorving it better, otherwise you can have as much as you want and you won't be absorbing enough, and wracking your kidney as ultimately they have to dispose of this mineral when not absorved.
Go on a diet low on fiber to increase absortion and high in fat but use good fats, like cocunot, olive, butter, ghee, lard. Also reduce or avoid dairy as they have lot of calcium that compite with magnesium for absortion, also pottasium is antagonist with magnesium so try to eat a low potassium diet for a while to see.