A field test and laboratory analysis program, including the four SE engine tests, was carried out to evaluate the comparative performance characteristics of a virgin and an acid/ clay re-refined API-SE/SAE 20W-40 automotive engine lubricating oil under selected operating conditions. The two oils had the same additive package. Eight new police patrol cars, four on virgin and four on re-refined oil, were run in 1979/80 under normal fleet operation and maintenance practices for 100 000 km. The fleet oil change interval was 5000 km. All engines were disassembled and rated at the end of their fleet service. There were no oil related problems. Engine vehicle parts showed normal wear and deposits. Some small differences, however, were noted in the used oil properties, engine sequence tests and field engine ratings. In general, both oils operated satisfactorily and were considered substantially equivalent.