The reduction of elastohydrodynamic (EHD) friction has become increasingly important to improve the efficiency of mechanical components. This study investigated the effect of friction modifiers (FMs) on full-film EHD friction using a ball-on-disc tribometer to elucidate the underlying mechanisms by which FMs affect full-film EHD friction. It was found that glycerol monooleate (GMO) lowered full-film EHD friction more than glycerol tris(2-ethylhexanoate) (GTE). This effect became more pronounced with increasing concentration of FMs. Neutron reflectometry (NR) and in situ reflectance spectroscopy suggested that GMO formed a thicker and denser adsorption film on surfaces than GTE, correlating directly with full-film EHD friction reduction. One of the possible mechanisms is that FMs, particularly GMO, adsorb on surfaces and may form oleophobic planes, where methyl groups are aligned in the outermost region of the adsorption film. A bulk base oil may flow more easily on such planes due to an interfacial slip than on steel surfaces. This reduces full-film EHD friction. The insights obtained in this study can be used to the design of lubricants that reduce full-film EHD friction. Graphical Abstract