The pilot said that he serviced the oil quantity prior to departure. The aircraft was in level cruise about eight miles from the airport when the pilot saw the oil pressure fluctuating, then begin decreasing, with a corresponding decrease in engine torque output. The pilot followed the checklist and shut down the engine when the aircraft was over the airport. The pilot set up a spiral over the airport to lose altitude in preparation for a power off forced landing. The pilot said he let the aircraft get too low while lining up for the runway and landed short in a field. FAA inspectors examined the aircraft and the engine at the site and found oil covering the right side of the lower fuselage. The oil filler cap was found loose in the filler neck. No evidence of oil leakage was noted to any part of the engine or oil lines. The 'O' rings in the oil filler neck were found in place and the filler neck was found attached to the engine with the correct torque.