You can improve gas mileage by getting an oil change.

Many of my customers ask me, if getting an oil change will improve gas mileage in their cars? My answers is you can improve your gas mileage by 1-2 percent if you use 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 due to increased friction inside the engine. Sometimes in older model cars, it is recommended to use heavier oil such as 10W-40 to compensate for wear and tear inside the engine, but we have to understand gas mileage will be affected.
Starting in 2011 some vehicles will take a different type of oil for their engines. The new ILSAC GF-5 oil specification is set to go into effect. One of the main drivers behind ILSAC GF-5 is improved fuel economy, the goal is to gain up to 0.5% improvement, which would translate into potential 250 million gallons of fuel savings and 450 millions by 2015 (according to Luis Guimaraes, Shell North America).
Another factor in the change are improved emissions system by reducing phosphorus volatibility, and improving catalytic converters life and reducing star-up emissions.
Is the job of the auto technician to talk to their customers and always use engine lubricants as recommended by the vehicle manufacturers.