Using an energy conserving or synthetic motor oil can also improve gas mileage, saving you money at the pump. By reducing engine friction, these oils can improve engine efficiency by 1 to 2 percent, which in turn improves gas mileage. Be sure to use the recommended oil viscosity range for summer, which may be a bit thicker than winter. Always refer to your vehicle owner’s manual for the recommended oil type and viscosity. Most vehicle manufacturers recommend 5W-30 oil for year-round protection.