Effects of typical engine-oil additives on peak traction coefficient and film thickness were evaluated on a disk-type traction machine. Film thickness was imputed from measured inter-disk capacitance.The addition of 1% ZDTP to an SAE 5 base mineral oil increased the imputed film thickness by some 130 nm or 15 percent. The addition of 1.5% detergent nullified this film thickness increase, which was, however, regained on adding 5% dispersant. A further increase of 150 nm was observed on adding 0.4 palmitic acid. From EHD film thickness and traction theory, changes in inlet oil viscosity or oil pressure-viscosity coefficient could not fully account for film thickness changes. It is concluded that the EHD film must be sandwiched between reaction films formed on contacting steel or cast iron surfaces. No corresponding film thickening could be found with chromium-coated cast iron toroids running against a steel disk.