The pilot had completed spreading the third load of herbicide on a paddock and was setting course for his employer's home base when the engine lost power. After conducting emergency checks engine power was restored. The mixture was leaned and a climb initiated while continuing to return to the strip. While the aircraft was still some distance from home the engine stopped. The pilot conducted a landing, downwind and downhill, across a contour bank in a wheat paddock. Examination of the aircraft by the owner disclosed that the aircraft had run out of fuel. The pilot had been using higher power settings and richer mixture settings than anticipated by the owner, who was monitoring the pilot's performance. The pilot had not landed into wind nor had he selected an appropriate landing area when faced with the need to conduct a forced landing.