I use Winsor Newton Liquin to mix with my oils for a quick drying medium.  Liquin improves the flow, transparency and smooths brushwork.  It is used for detail, blending and glazing.  It also has a good resistence to yellowing.

I use Winsor Newton oil paints and consists of the color pigment plus linseed and safflower oils. 
The Grumbacher and Gamblin oil paints that I use is color pigment plus alkali refined linseed oil.