Artists Oil Colors can be expensive.  If you want a stable, less expensive way of extending your Artists Oils, consider using Gamblin’s 1980 Oil Colors, their student grade colors, alone or with your regular palette of colors.  According to Dave, this is more stable than extending with things like marble dust (calcium carbonate), etc.  Also, for colors you may like that are a very intense, such as Pthalo blue or any staining pigment, some artists prefer to use the student grade version because the reduced pigment load is less overpowering (and less expensive).