The pilot reported that prior to landing all landing gear system indications were normal, with all legs of the gear indicated down. After touchdown the left wing slowly lowered and the pilot was unable to prevent the wingtip and propeller from contacting the ground. The aircraft then veered to the left off the runway with the left landing gear collapsed. Subsequent examination found a torsional overload failure of the left maingear leg torque tube originating from cracks near the fork bolt boss. The torque tube cracking is thought to be caused by repeated overloading of the torque tube assembly due to inadequate lubrication, during routine maintenance, of the retraction linkages. This is a known problem and is addressed in Airworthiness Advisory Circular (AAC) 160-4 of February 1985. The subject torque tube was of an old design without the later modification introducing the reinforcing gusset.