Buy special electric motor oil
Whenever you have to lubricate an electric pump, bench grinder or other electric motor, don't pull just any oil off the shelf. Instead, pick up electric motor oil, like 3-In-One electric motor oil (about $3 at amazon).
Don't use automotive motor oil! It's too “thin” for electric motors. Plus, it contains detergent. If the excess oil seeps onto the motor windings, it might dissolve the insulation and fry the motor.