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. Whale oil was formerly used as a friction modifier in automatic transmission fluid.
This additive is sometimes needed in limited slip differentials.
References
Friction modifier Wikipedia(Text) CC BY-SA