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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.