Gasoline is a varying blend of aromatic and aliphatic hydrocarbons with alcohols at varying levels being used to decrease oil consumption. Alcohols are very oxidizing and can cause swell.
Standard FKM materials are used for non-blended gasoline, while high-fluorine FKM is needed for blended fuels. NBR and HNBR can be used for all blends.
A check of the General Properties of O-Ring Elastomers chart in the Material Selection Guide shows that Teflon™, Fluorocarbon and Epichlorohydrin possess enhanced resistance to exposure to aromatics, aliphatics and alcohols, over a working temperature range suitable for automotive use. Check with an Apple Rubber representative for biofuels and high-methanol fuels.
Additionally, Nitriles, specially compounded to reduce swelling in gasoline, are sometimes employed for automotive use.