The study was undertaken to investigate the difference between theoretical and actual, shorter service-lives of vehicle engines and to determine means of prolonging the service-lives. Test-stand experiments were run with trial engines and measurements of engines in urban service for buses and trucks were made over extended operating intervals. Testing procedures applied spectrometric analysis to the engine oil in order to examine impurities.Results indicated that the greatest portion of impurities in engine oil originated outside of the engine; generally, 70% or more of oil impurities were of this source, the balance being produced by wearing processes in the engine. This result points to the need for better air filtration systems, which are believed to offer prolonged service-lives. The paper establishes a design goal of a maximum 70% impurities from exterior sources since the proportion was higher in many engines.