Gamblin Artists Colors: Interactive Mediums Guide Great interactive tool for learning about and selecting oil mediums
Medium - Adding linseed oil alone to a color will increase the flow but won't give the sharpest edges. Adding thinner alone gives sharp edges but a chalky appearance. A medium of HALF CITRUS THINNER AND HALF LINSEED OIL is best for most situations.
If your oil painting is sinking in, "Oiling Out" comes to the rescue. This how to video includes a dialogue on the difference between "Oiling Out" and using .