Friction modifiers are added to lubricants to reduce the surface friction of the lubricated parts. Typically these are polar chemical compounds having high affinity for metal surfaces and possessing long alkyl chains. Glycerol mono-oleate is a common example of a friction modifier. This additive is needed in limited slip differentials. Glycerol is an organic compound, also commonly called glycerin or glycerine. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydrophilic hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol substructure is a central component of many lipids. Glycerol is sweet-tasting and of low toxicity. In foods and beverages, glycerol serves as a humectant, solvent and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods (e.g., cookies), and as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of leaves. As a sugar substitute, it has approximately 27 calories per teaspoon and is 60 percent as sweet as sucrose. Although it has about the same food energy as table sugar, it does not raise blood sugar levels, nor does it feed the bacteria that form plaques and cause dental cavities. As a food additive, glycerol is labeled as E number E422. Glycerol is also used to manufacture mono- and di-glycerides for use as emulsifiers, as well as polyglycerol esters going into shortenings and margarine. It is also used as a humectant (along with propylene glycol labelled as E1520 and/or E422) in the production of snus, a Swedish style snuff that the Swedish government subjects to the same regulations as "food" because it is used orally.