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The interest on improving fuel efficiency of vehicles is increasing day by day. Fuel efficiency standard for diesel commercial vehicles such as buses and trucks was published in Japan. Using a fuel efficient engine lubricant is one of the effective paths and there are several 5W-30 diesel engine lubricants in Japanese market which are advertised to give a benefit on fuel efficiency against 10W-30 oil. During the development of 5W-30 fuel efficient diesel engine oil, it was revealed that the piston underside was significantly blackened by the detergency engine test (JASO M 336: 2014). In this paper, the causative agent which blackened the piston underside was investigated and the formulation to inhibit this blackening phenomenon was studied. Through several tests, it was considered that use of poly methacrylate based viscosity index improver and ester type friction modifier deteriorated detergency performance.