Oiling a composition bearing, like the upper sleeve bearing on a Convertible, is harmful to the bearing because as the machine runs, the friction of the armature shaft at the point of contact with the bearing will turn the oil into a 'resin' (like on the bottom of a pot). The words "do not oil" are printed on the top metal bearing plate.
From the Hoover Convertible owners manual, printed 9/76, on page 18 under the heading "Lubrication": "The motor is equipped with one ball bearing and one sleeve bearing. The ball bearing should be lubricated, but the sleeve bearing should never be lubricated".