THE CERTIFICATED PRIVATE PILOT AND A PASSENGER DEPARTED ON A LOCAL AREA FLIGHT. WHEN THE AIRPLANE REACHED ABOUT 400 FEET ABOVE THE GROUND, THE ENGINE SUDDENLY BEGAN TO RUN ROUGH AND LOSE POWER. THE PILOT EXECUTED A 180 DEGREE TURN TO RETURN TO THE AIRPORT. WHILE ON FINAL APPROACH THE PILOT NOTICED AN AIRPLANE LANDING ON THE RUNWAY AND ANOTHER OCCUPYING THE TAXIWAY ON WHICH HE HAD PLANNED AN EMERGENCY LANDING. THE PILOT EXTENDED HIS APPROACH AND LANDED IN A FIELD EAST OF THE AIRPORT. THE AIRPLANE WAS SUBSTANTIALLY DAMAGED AND THE OCCUPANTS RECEIVED MINOR INJURIES. NO MALFUNCTION OF THE ENGINE WAS FOUND. AN EXAMINATION OF THE FUEL REVEALED THAT FUEL IN THE RIGHT FUEL TANK WAS CONTAMINATED WITH AN UNDETERMINED AMOUNT OF A LUBRICATING OIL-LIKE SUBSTANCE. NO ENGINE OR FUEL SYSTEM PROBLEMS INVOLVING FUEL OBTAINED FROM THE FUEL VENDOR WERE REPORTED.