Fullerenes (FLNs) have attracted attention as lubricant additives because of their antioxidant properties and potential to reduce friction and wear. However, the mechanisms by which FLNs function in lubricating oils remain unclear, particularly with respect to their existence forms in oil and interactions with sliding surfaces. In this study, the tribological properties of FLN-containing oils were investigated in relation to the dispersion state of FLN molecules to clarify the mechanisms underlying their effects on friction and wear. The dispersion state and existence forms of FLNs were characterized using UV-Vis absorption spectroscopy and small-angle X-ray scattering (SAXS), while their interactions with solid surfaces were evaluated using neutron reflectometry and atomic force microscopy. Tribological properties were investigated using reciprocating and ring-on-plate friction tests, and friction under microscopic surface contacts and nanoscale-gap conditions was further examined using a nanoscale-gap friction apparatus. UV-Vis absorption spectroscopy and SAXS revealed approximately 10 nm aggregates in oil containing 1000 ppm FLN. The absence of UV-Vis spectral shifts associated with direct intermolecular contact suggested that these aggregates consisted of FLN molecules coated with hydrocarbon molecules derived from the base oil. Neutron reflectometry and atomic force microscopy indicated negligible adsorption of these aggregates onto solid surfaces. Nevertheless, oil containing 1000 ppm FLN reduced friction and wear, whereas oils containing 10 and 100 ppm FLN increased friction and wear. Additional evaluations showed that the aggregates did not directly reduce friction through rolling or sliding actions. The increased friction and wear at low FLN concentrations were attributed to abrasive interactions of dispersed FLN molecules with the sliding surfaces. In contrast, the friction and wear reduction at 1000 ppm FLN was attributed to enhanced oil-film retention under boundary lubrication conditions rather than direct friction reduction by the aggregates, surface adsorption, or tribochemical reactions. These findings provide new insight into the tribological mechanisms of FLN additives and demonstrate that friction and wear reduction can be achieved through improved oil-film retention without relying on adsorption or tribochemical reactions.