The frictional behaviors of soybean oil and heat modified soybean oils with different Gardner scale viscosities as additives in hexadecane have been examined in a boundary lubrication test regime (steel contacts) using Langmuir adsorption model. The free energy of adsorption (ΔGads) of various heat modified soybean oils were compared with unmodified soybean oil. It was observed, that relative distribution of unsaturation in the molecule, average molecular weight and thus viscosity can affect adsorption on the metal surface. Heat modified soybean oils consistently have a lower ΔG ads values compared to unmodified soybean oil. Lower ΔG ads corresponds to stronger adsorption on the metal surface. The heat modification thus provides oils with stronger adsorption and higher viscosities, good for use in industrial lubricant formulations. This information can be used to design suitable lubricant molecules that will have optimum structure for effective metal adsorption as well as exhibit excellent boundary lubrication properties.