You can improve your gas mileage by 1%–2% by using the manufacturer's recommended grade of motor oil.
1%–2%; Equivalent Gasoline Savings: $0.04–$0.07/gallon
Also, here's another good tip: look for motor oil that says "energy conserving" on the API performance symbol to be sure it contains friction-reducing additives.
More can be found on the EPA's fuel economy source: http://www.fueleconomy.gov/feg/maintain.jsp
Be sure to call AAMCO for all your car care needs: 1-800-GO-AAMCO or visit your nearest AAMCO.