Used Engine Oil refers to lubricating oil that has been drained from engines after it has served its purpose of reducing friction, cooling, and cleaning engine components. Over time, engine oil becomes contaminated with metal particles, dirt, combustion residues, and degraded chemical additives, which reduce its effectiveness. Sources of used engine oil include vehicles (cars, trucks, buses), industrial machinery, marine engines, and generators. It is classified as hazardous waste due to its toxic components and must be carefully managed.