<p>Organic friction modifier additives, such as fatty acids, are widely used in the boundary lubrication regime to reduce friction. In this study, we employed resonance shear measurements to study 0.1 wt% solutions of branched-chain fatty acids in a linear model base oil/hexadecane and branched base oil/poly(α-olefin) (PAO) confined between mica surfaces under various applied loads (<i>L</i>). At <i>L</i> = 3.3 mN, the viscosity parameter (<i>b</i>_s) values of isostearic acid and isostearic acid T in hexadecane were one-seventh and one-thirtieth of the value of pure hexadecane, respectively. Especially, isostearic acid T in hexadecane had a lower viscosity parameter <i>b</i>_s value, one-eighth that of pure hexadecane even at <i>L</i> = 22.7 mN. On the other hand, the <i>b</i>_s value at <i>L</i> = 3.3 mN of isostearic acid in PAO was half the value of pure PAO and this effect disappeared at <i>L</i> = 11.6 mN; isostearic acid T in PAO resulted in a behavior similar to that of pure PAO. The densely packed structure of linear lubricants as hexadecane confined between the surfaces is known to cause a high <i>b</i>_s value and such a structure could be disturbed in the presence of branched-chain fatty acid additives. In case of the branched base oil, such as PAO, the densely packed state was not formed under confinement, thus the effect of the branched-chain fatty&#xa0; acid additive was not significant. This study demonstrated that a relatively small difference in molecular structure of an additive is important for efficiently reducing friction, especially in the boundary lubrication regime of linear base oils.</p> Graphical Abstract <p></p>