Use the Recommended Grade of Motor Oil

You can improve your fuel mileage by 1-2 percent by using the manufacturer's recommended grade of motor oil.
For example, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage by 1-2 percent.
Using 5W-30 in an engine designed for 5W-20 can lower your gas mileage by 1-1.5 percent.