Only when it is used neat to thin the paint. Adding it to oil paint will create a thinner, faster-drying layer that is great for underpainting, so that your next layers will be thicker and fatter. But if it is part of a medium you can discount it because it evaporates and you are left with the same amount of oil that you had before.

Any medium that contains any amount of oil that you then add to your oil paint, will add more oil to the paint than it contained before. A lean medium might be 10 parts solvent to one part oil. But adding it to your oil paint still increases the oil content.

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