The pilot reported that he had 20 gal of fuel on board when he took off. He climbed to 10,000 ft for a parachute jump, then returned to the airport with an estimated 10 gal of fuel. During a downwind landing, he elected to go around. He said the engine lost power as he was climbing through about 75 ft agl. A forced landing was made in a small pasture. Initially, the mishap was reported as an incident. Before the aircraft was examined, wings had been removed and the aircraft and wings had been transported from the accident site. When examined, both wing tanks were empty, except a small amount of 100 low lead (blue) fuel was found in the right wing tank. Fluid taken from the lines to the left wing tanks had the appearance of automotive fuel. The owner reported that automotive fuel had previously been used. No fuel was found in the carburetor.