Shortly after commencing a descent, and several minutes after switching from the 'aux' to 'main' fuel tanks, the fuel pressure dropped to zero and the engine quit. The pilot was unable to accomplish a restart and executed a forced landing. Upon breaking out of a 500 foot overcast during dusk conditions and over an occupied road and wooded terrain, the pilot maneuvered the aircraft away from the road towards the only suitable landing area available. The aircraft collided with trees during this maneuver. Post crash examination by the FAA revealed no mechanical malfunction within the fuel system. The pilot reported departing Moses Lake, Washington, with 20 gallons of 80/87 octane fuel in the 'aux' fuel tank and a 50/50 mix of 100 low lead and automotive gasoline in the 'main' tank. Research into the issuance of an STC for automotive gasoline in the Navion was not pursued due to a finding of 'severe vapor lock' during its use.