Friction Reduction for Cam and Follower Requires Optimized Combination of Surface Treatment, Oil, and Additives: Engine oil lubricant additives have been modified to reduce their negative impact on such aftertreatment devices as NOx reduction catalysts and diesel particulate filters. Anti-wear and anti-oxidation additives such as the phosphorous and sulfur in ZDDP and detergent additives including calcium and magnesium, for example, are the lubricant additives that have caused concern about their influence on aftertreatment devices. Thus, recently developed engine oil contains lower fractions of sulfated ash, phosphate, and sulfur (low SAPS). On the other hand, the friction generated by sliding components needs to be reduced to improve fuel economy. Surface engineering has advanced through the development of coating materials that protect interfaces of sliding components. The engine oil for these components has modified properties for reduced deterioration of aftertreatment devices. With these backgrounds, researchers at Kyushu Industrial University and Nippon Piston Ring, investigated the friction characteristics of a direct-acting cam and follower assembly of a valve train.