Twist compression tribotester (TCT) and 4-ball extreme pressure (EP) methods were used to investigate commercial polysulfide (PS) and biobased polyester (PE) EP additives in paraffinic (150N) and refined soybean (SOY) base oils of similar viscosity. Binary blends of EP additive and base oil were investigated as a function of additive concentration. In addition to weld point (WP), 4-ball EP produced a set of preweld data, notably peak torque and wear scar diameter, which were found to correlate with WP results. TCT gave a 5-fold larger time-to-failure (TTF) for neat SOY than for neat 150N, whereas 4-ball EP gave similar WP (120 kgf) values for both neat oils. This difference was explained by invoking boundary contribution to TCT but not to 4-ball EP method. Both additives improved the WP and TTF of the base oils, which further increased with increasing additive concentration. However, the extent of the improvements was highly dependent on the chemistries of the additive and base oil of the blends. Thus, at similar concentrations, the WP of PE was higher in the 150N than in the SOY base oil, while the WP of PS was higher in the SOY than in the 150N base oil. Similarly, TTF of 150N was higher with blended PE than PS; whereas for SOY, it was higher with blended PS than PE. This chemistry effect was attributed to relative compatibility between EP additives and base fluids. The results suggest that a substantial reduction (up to 4-fold) in EP additive usage in commercial lubricant formulations can be achieved through proper selection of compatible base fluids and additives.