And why would oil just drained from a prized Corvette or high tech turbocharged Ford engine suddenly not be suitable for simple handgun? It was the perfect lubricant for a very demanding application for months and thousands of miles of continuous service. Should be fine for another hour's cumulative service in a handgun seems to me.
Joe
Joe, you are correct. The lubrication properties of any modern motor oil exceed any conceivable stress a cycling firearm would place on it. In fact, in the early days of the GWOT some military units in Iraq were experiencing shortages of CLP (Break Free) - the supply system just couldn't get it to them fast enough. The weapons technicians at Rock Island Arsenal recommended motor oil as a perfectly good substitute. They had just a few cautions - first, motor oil is just a lube and is a lousy cleaner (they actually recommended transmission fluid as a substitute cleaner because of its detergent properties). Next, motor oil has lower viscosity and lower adhesion properties than most gun lube, so it tends to run off more readily and may not do a good job as a rust preventative.
Evan Marshall often states that right after 9/11 he was hired as a contractor by the DOE to run training programs for their nuclear site security teams. They ran hundreds of thousands of rounds through thousands of M-16s and M-4s using Mobil1 as the only lube. They would just swing by a local auto parts store and pick up a case of oil before heading to the range. He reports that they never had a single lube related weapons failure.