Used oil is defined in Title 40 of the Code of Federal Regulations as "any oil that has been refined from crude oil, or any synthetic oil, that has been used and as a result of such use is contaminated by physical or chemical impurities."

Used oil is typically contaminated or mixed with dirt, fine particles, water or chemicals, all of which affect the performance of the oil and eventually render it unusable. Used oil does not include products derived from vegetable or animal fats or petroleum distillates used as solvents. Antifreeze, cleaning agents, gasoline, jet and diesel fuels are not used oil.

• USI's Used Oil Program (pdf)
• U.S. EPA Used Oil Management Program (external link)