Well, let's clear 2 things : 1. You will not get dark because you don't use soap. 2. If your face gets extremely dry even after one wash with plain water, it is very unlikely that the sun makes it 'oily'. Your face might be getting sweaty. As to your basic question : unfortunately there is no quick fix/single solution to dry skin. And looking at all your health issues I sincerely suggest up visit a dermatologist personally. A detailed question answer session is needed in your case to understand your basic skin type and then treat it accordingly. Do not use creams suggested by non dermatologists.