Rod bearings are actually lubricated by a hydroplaning process which keeps a film of oil between the crank and the bearing shells. During periods of lugging an engine this hydroplaning effect is broken down, and the film of oil breaks down and of course the upper shell shows the most wear. An engine that has remarkably more wear on the upper shell has been subjected to a lot of lugging as compared to bearings showing normal wear. It was nothing unusual for old timers to keep a vehicle in high gear until it started to buck, or they shifted right before they thought the motor was going to die.