This is a difficult one. The reason there are so many mediums available is that they offer different results and are used for different purposes. In addition to this, you can create your own oil medium recipe to ensure that you achieve the result you are looking for. This all requires time, patience, and creativity to figure out. The first thing to do in it all, however, is to decide what it is you want to achieve. This will guide the way in terms of what medium to try and how to further enhance your paint.