Used oil is generally defined as oil that no longer meets the original requirements (e.g. oily) due to a change in properties and whose properties cannot be further enhanced by treatment. Potential sources are used oils from engines and gearboxes, oily emulsions, bilge water, tank residues etc. These waste oils have different densities, viscosities and impurity levels.

Used oil can be treated or refined by chemical-physical methods or used as a premium fuel for the cement industry or smelters. The Waste Oil Regulations stipulate that, subject to technical, economic and organisational constraints, waste oil should be treated materially by refining as a matter of priority. The composition of the used oil and the level of contaminants in the used oil likewise determine the disposal method.