Molecules with open reactive sites that do not have their full complement of hydrogen are called unsaturated . This is the same saturation terminology used to describe edible fats and oils. Fully saturated base oils are known for their light color, low odor and strong resistance to chemical attack. Less saturated base oils are darker in color, have stronger odors and are less stable when used in engine oils. In a refinery, base oils are stabilized by processes that increase the degree of saturation and remove sulfur and nitrogen.
Solvent extraction is an older, less effective process.
The solvent extraction process involves mixing the oil with a special solvent that can be dissolved in water. The solvent can also dissolve some unsaturated oils. The mixture is allowed to separate into two phases, similar to an oil and vinegar salad dressing. The top layer is purified oil. The bottom layer contains the solvent plus some of the unsaturated molecules.