Spontaneous low-speed pre-ignition, strong knock and other abnormal combustion events that occur in supercharged direct-injection engines are viewed as serious issues. The effects of the engine oil and the components of engine oil additives have been pointed out as one cause of such abnormal combustion. However, the mechanisms involved have yet to be elucidated, and it is unclear how the individual components of engine oil additives influence autoignition.This study investigated the effect on autoignition of boundary lubricant additives that are mixed into the engine oil for the purpose of forming a lubricant film on metal surfaces. A high-speed camera was used to photograph and visualize combustion through an optical access window provided in the combustion chamber of the four-stroke naturally aspirated side-valve test engine. Spectroscopic measurements were also made simultaneously to investigate the characteristics of abnormal combustion in detail. We mixed Zinc additive into primary reference fuel (PRF50) at zinc concentration of 115 ppm, 570 ppm and 800 ppm. In addition, we mixed Molybdenum additive into PRF 50 at molybdenum concentration of 140 ppm, 700 ppm and 980 ppm. Combustion experiments were conducted in this condition.The experimental results revealed that both the Zinc and Molybdenum additives had little effect on abnormal combustion.