Increasing numbers of vehicles equipped with downsized, turbocharged engines have been introduced seeking for better fuel economy. LSPI (low speed pre-ignition), which can damage engine hardware, is a potential risk of the engines. We reported that engine oil formulation affects frequency of LSPI events, and formulating magnesium detergents into oil is a promising option to prevent LSPI events. From the viewpoint of achieving better fuel economy by engine oil, lowering viscosity is being required. However, it causes reduced oil film thickness and will expand boundary lubrication condition regions in some engine parts. Hence, a technology to reduce friction under boundary lubrication becomes important. To establish technology to reduce friction in low viscosity oils while ensuring good LSPI prevention performance, effective use of molybdenum dithio-carbamate (MoDTC) with magnesium detergents was investigated, since MoDTC is known as one of the most effective friction modifiers to reduce friction under boundary lubricating condition. Our friction study revealed that magnesium detergent deteriorates low friction performance of MoDTC. Throughout a series of XPS analyses, we clarified that magnesium detergent scraped poly-phosphate tribofilm derived from zinc dithio-phosphate (ZnDTP) on the sliding surface, which inhibited MoS2 tribofilm formation, causing high friction. Borated dispersant was formulated to oil to promote formation of harder poly-borophosphate tribofilm which prevents itself from being scraped by magnesium detergent, causing low friction. This formulation technology of engine oil is applicable for future ILSAC GF-6 engine oil and after, where LSPI prevention performance and low viscosity are required for excellent fuel economy.