While in cruise flight, the pilot noted that the left engine began losing power and oil pressure. He stated that he then shut down the engine and feathered the propeller. He declared an emergency and descended to land. While on final approach, he lowered the landing gear and selected full flaps to slow the aircraft. He stated the airspeed decayed and the aircraft began to roll and yaw to the left. Subsequently, it contacted the ground in a left wing low attitude, then partially cartwheeled before coming to rest. A post accident examination of the left engine revealed that a turbo-charger oil seal had deteriorated and failed, allowing oil to escape through the turbocharger exhaust. There was evidence of the beginning of progressive failure of the turbocharger. Also, there were indications that the left propeller was not fully feathered and that it was windmilling at impact. The pilot believed that he may have moved the left prop control out of the feather position by mistake.