Modern lubricants such as fuel economy engine oils control friction through tribofilms formed by functional additives mixture. Although many cases on synergistic or antagonistic effects of additives on friction have been reported, their mechanisms are poorly understood. Here we found that some of the detergent additives with metallic soap reduced friction synergistically with molybdenum dithiocarbamate (MoDTC), which is a widely-used friction modifier additive that forms slippery tribofilm, while detergents themselves increased friction without MoDTC. These results indicate that detergents enhance friction-induced formation of slippery tribofilms by MoDTC. To reveal this mechanism, using surface force apparatus equipped with a resonance shear measurement system (SFA-RSM), we examined mechanical properties of detergent-containing oils confined at single-asperity contact. Then we found that detergents increased minimum narrowness of confinement space producing interfacial repulsive force and shear resistance, which we called steric bulkiness. Moreover, friction coefficient when these oils mixed with MoDTC was very well correlated with steric bulkiness. Dynamic light scattering experiments showed that steric bulkiness possibly originated in the size of secondary grains including plurality of primary grains of detergents. We also examined the effects of non-detergent nanoparticles based on colloidal silica or carbon black. These nanoparticles reduced friction synergistically with MoDTC and also increased steric bulkiness in the same manner as detergents. These results suggest that steric bulkiness enhances friction-induced tribofilm formation. We consider that microscopic friction between nanoparticles and solid surfaces helps MoDTC receive friction energy to form tribofilms, resulting in synergistic friction reduction. Steric bulkiness of nanoparticles may be applicable to control many other tribochemical reactions.