I have a collection of medium old (in my opinion) radios such as a ts-440, ft-920, ft-840 and on all of them the vfo/tuning knobs are very stiff and "stutter" when turning. These radios have been in storage for a long time though they otherwise work.
Old plastic can crack into tiny bits, ruining everything; however yours doesn't sound that old.
Ideally, silicone grease applied to moving surfaces is the best; it doesn't degrade plastic, lots of things do degrade plastic. Apply to shafts after removing them if that seems best, don't use too much.
Although silicone grease (sold for putting on spark plugs among other things) doesn't harm plastics as far as I know, silicone sprays contain nasty solvents.
Tension adjustments are probably different from radio to radio, think old fashion stringing diagrams.
Be careful when loosening and tightening knob screws, the can crack plastic on the way in or out.
