These are one group of machines that was made to last 30 years, and they hit it quite accuratly.
They came with a "self lube" inbedded in the gears, and over time this gets gooey and hardens up. So at some point around the 30 year mark, the machine needs to be thoroughly cleaned and every bit of that goo removed. Usually takes solvent and a blow dryer. If you just soften it, it will harden back up. Once it's gone, then the machine needs to be oiled regularly as it no longer had the self lube ability.
And since they were synthetic gears, once it starts to stiffen up, it's super easy for someone to try to force a knob and snap something internally. And like said above, the main cam stack would crack. This is something even though they also have external cams, though the only one I'm real familiar with is the 6570.
Do NOT attempt to force any button, knob of lever. You'll break it before you free it up, and these are not easy machines to work on.