Almost all gasoline today includes the additive ethanol, which is added to fuel to oxygenate it and reduce air pollution. When choosing gasoline at the pump, you may be given a choice between gasoline rated E10 (which is 10 percent ethanol) and E15 (15 percent ethanol). To protect your car, Popular Mechanics suggests avoiding gas rated E15 if your car was manufactured before 2001. Ethanol is an alcohol, and it can be corrosive to rubber and certain metals, and many vital parts of cars are made from those materials. Cars made after 2007 are compatible with the E15 blend because automakers adapted the newer cars for the higher grade of ethanol gas.