Milk of magnesia
is magnesium hydroxide
which is basically used as an antacid( something like digene or mucaine gel). Frankly as a dermatologist
i would not suggest it for the skin problems you have mentioned as it would hardly be of any help to you. What I gather is that you basically need a good skin care
regime suiting an oily skin. Cleansing, toning(optional) and moisturising is the rule. Firstly, use a good face wash meant for oily skin or acne prone skin( containing salicylic acid
). If using a toner(avoid it in winters), use an alcohol free one then follow it with a water based moisturiser(as they are specifically meant for oily skin). One should always use a good sunscreen as it will prevent further occurence of dark spots. For your skin use a sunscree in gel base. Make it a point to atleast wash your face thrice in a day. drink good amount of water, eat fruits and stay happy and healthy. Take care.