In recent years in Japan, there is growing interest in high performance diesel engine oils to control oil consumption and extend engine life. As a background to this trend, there is, a) the prevalence of turbocharged engines which are mounted on nearly 50% of the highway heavy duty trucks and, b) the severe conditions due to continuous high speed operation under heavy load which has resulted from the development of the highway network. Under the above conditions, the thermal load of the piston is higher. In addition, there are strong requirements from users for extended engine durability, e.g. running 700,000 Km to 1 million Km before engine over-haul.To cope with these requirements, it has become necessary not only to improve the engine design but also to enhance the performance of the diesel engine oil. This paper presents a fundamental study of the effect of engine oil formulation on engine durability, primarily oil consumption, in a laboratory dynamometer test using a MITSUBISHI 6D22T turbocharged diesel engine, which is mounted on 20 ton G.V.W. heavy duty trucks.