The dependences of changes in the contaminant contents and base numbers of motor oil during refining are established. Finely disperse contaminant particles are enlarged if a mixture of ammonium hydroxide with 1% urea is added to working engine oil. Subsequent centrifugation with cleaning agents incorporated into the lubrication system allows almost all contaminants to be removed from the oil. The dependences of the content of insoluble precipitate and base number on the operating time in the engine of the motor oil with the added reagent is established. Periodic cleaning of engine oil by the proposed method allows the service life until replacement to be extended and the operational characteristics of the internal combustion engine to be increased.