I have an IBM Selectric III that has bee...

Typically a machine that has been out of use, but was previously working may just need some lubrication on the motor centrifugal clutch assembly, but the entire machine probably would need lubrication also. You can first try to hand cycle the machine by grabbing the shaft that the carrier travels on and rotating it towards the (top to rear) rotation. This shaft rotates on revolution per print cycle and at rest the slot is slightly towards the rear of the machine. If you must lubricate the machine, then remove the cover. Lift the lid, place each hand of platen knobs and with thumbs, push down on release latches, after platen is out of machine, reach inside left and right cover and release the top cover latches and pull the cover straight UP to remove. Use 10 weight oil on motor bushings and centrifugal clutch hub (motor shaft).