Energy and environment are two most talked about words by environment activists and organizations. The oil which is used for moving parts and other friction induced machines, in order to reduce friction and save energy becomes contaminated after one use. The reuse of such oil may not give desirable results. Disposing and reuse are two possible alternatives. The disposal of the waste oil has serious environmental foot prints. It is envisaged to reuse the oil as far as possible. Many investigations are reported on reuse of oil and re-refining of used oil. Many methods such as adsorption, dehydration, vacuum distillation, solvent extraction have been investigated for the re-refining of used oil. Current review summarizes research and studies of used oil.