Oiled bearings? They are sealed zz bearings. By the time those bearings would need service, you can probably consider the rest of the $30 tool ready to replace. The ball bearings ride against the 2-piece thermo-formed plastic tool housing. Not exactly a precision instrument.
With the manicure rotary tools, I do not see ANY fan and the whole tool looks sealed and probably water tight. They could be high rpm and low torque. Probably have no fancy gearbox either.