Fuel efficient SAE 5W-20 automotive gasoline engine oils with molybdenum dithiocarbamates (MoDTCs) type friction modifiers were developed. The additive formulations were optimized with a motor-driven valve train friction test. The valve train friction was measured for both new oils and used oils, which had been prepared in a fired operation with bench engines, in order to evaluate the durability of the fuel efficiency. The combinations of MoDTCs, zinc dithiophosphates (ZnDTPs) and calcium salicylate type metallic detergents were found to be effective in reducing valve train friction and in improving its durability. From the results obtained, two oils, which had different ZnDTP formulations, were manufactured for evaluating field test performance. The SAE viscosity grade of the two oils was set at 5W-20 out of consideration of the reduction of hydrodynamic friction.Field test performance was evaluated with vehicles used for commuting in the Yokohama area. It was concluded that the wear and oil consumption, which had been a major concern in the use of low viscosity oils, of the developed SAE 5W-20 oils were at almost the same level as API SG, SAE 10W-30 and 5W-30 oils. Regarding the ZnDTP type, primary ZnDTP rich oil was superior to secondary ZnDTP rich oil in terms of the oil degradation. The friction coefficients of the used oils in the field test measured with a reciprocating friction test did not show any substantial difference due to the ZnDTP type. The fuel efficient durability of the developed oils was around 7,500 km.