[john]

Your local gas station probably offers you 3 levels of gasoline, like
gold, silver and bronze. These typically have different levels of
octane in them and you will generally get a little better mileage for
the more expensive type.

In addition to different octane levels there are many different
additives that can be in a companies brand of gasoline and this
differentiation can have a big impact on your vehicle.

One type of additive is a detergent which generally cleans your
engine. Without some level of detergents engine deposits form and this
causes reduced fuel efficiency, acceleration and power as well as
increasing emissions and rough idling.
http://www.zone-car.com