This paper describes engine rusting obtained in three closely controlled, “Aunt Minie” passenger-car, field services tests. Under severe conditions imposed both by the mode of operation and winter weather, appreciable engine rusting was observed. In addition to the major effect of lubricating oil on rusting, other important influences were fuel scavenger and operational variables such as ambient temperature, engine temperature, and crankcase ventilation. The range of motor oil antirust performance found in severe field service appears to be inadequately predicted by existing laboratory engine rust tests. Better definition by laboratory tests is desirable to permit selection of motor oils of adequate antirust performance.