On May 9, 1998, at 1105 mountain daylight time, an Aero Commander 500-A, N9382R, registered to and operated by Corporate Air as a 14 CFR Part 91 positioning flight, landed at the Billings Airport, Billings, Montana, with the left-main gear in the unlocked position. The left main gear collapsed during the landing roll. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft was substantially damaged and the commercial pilot, the sole occupant, was not injured. The flight had departed Billings at 0810, and was destined for Minot, North Dakota. The pilot reported that after takeoff from Billings, the left-main landing gear would not fully retract. The pilot extend the landing gear and performed various emergency procedures for about three hours in an attempt to fully extend the left-main. The pilot was unable to get a down-and-locked indication on the left-main gear and elected to land the airplane with the right-main and nosewheel extended. During the landing roll, the left-main landing gear collapsed and the airplane slid down the runway. Inspection of the left-main landing gear by an inspector from the Federal Aviation Administration Flight Standards District Office, Helena, Montana, revealed that the upper-main landing gear strut's outer-body to inner-body needle bearing had failed and the lower needle bearing was near failure. The lack of lubrication in this area was noted, along with evidence of severe corrosion development. The right-main landing gear was also inspected, and during the disassembly, similar conditions existed for the right gear as was found for the left. The Manufacturer's Inspection Program for the 500 series aircraft does not address a calendar and/or time inspection interval, or lubrication interval for the upper and lower needle bearings. The manufacturer's inspection program maintenance manuals for the 600 and 700 series aircraft, which have the same design landing gear assemblies, identify procedures to overhaul and lubricate the landing gear at a five year or three thousand hour interval.