The shaft is oiled by crank case oil mist getting in the cavity at the bottom of the housing and carried up the shaft in the spiraled groove. Dirty oil gums it up and the oil turns to varnish. Too bad they still don't have the external oil tube. I guess it just cost too much for the bean counters. If the gear is too hard too remove, as some steel gears are, soak the whole bottom portion in lacquer thinner turning the shaft occasionally. Soon the varnish will dissolve. Replace it with oil from the bottom.