I was just asked this question and thought everyone would like to be aware of the answer.
You should not used medium clear. You negate being able to use water with oil paints that you put on top of this medium. For best results, only use water or medium specifically designed for water soluble oil paints. The label will be very specific on what types of paints its to be used. Also, brands can be mixed. I often have several tubes of water soluble oil paints from different vendors.