About 10 minutes after departing Bankstown for a local flight, the pilot noticed a reduction in engine RPM. Believing the throttle friction had become loose, he increased the throttle setting and tightened the friction. Soon after, the RPM again decreased so the pilot commenced to turn back towards the airport. During the turn the engine oil pressure indication decreased to zero. The pilot advised the tower of the problem, and of his intention to land on an abandoned airstrip. The engine was shut down after it began to vibrate, and the pilot informed the tower that a forced landing would be carried out in a field. During the subsequent landing the nosewheel entered a drain and the aircraft overturned. Investigation later revealed that a connecting rod bolt had failed, resulting in a loss of oil pressure and subsequent engine failure.