The pilot reported that the engine suddenly began to run rough and backfire during a pipeline patrol flight at 300 ft AGL. The roughness worsened and the pilot made a forced landing on a dirt road. During rollout, one of the wings caught vegetation bordering the road and the airplane veered off the road into a ditch. Examination of the airplane revealed that it was supplemental type certificated for the use of automotive gasoline, and that it was fueled with auto gas. Examination of the Lycoming O-290-D2 engine revealed that the #4 exhaust valve was stuck open, and heavy carbon deposits were present on the valve. The valves in the #3 cylinder were found to be burned. Lycoming does not recommend the use of auto gas in Lycoming aircraft engines. Service Letter L199 notes that gums can form in auto fuels which can cause sticking valves. FAA AC 91.33A contains several cautions on the use of the auto fuel concerning quality control, gum formation, volatility, and carburetor icing.