When that selector starts to strip, gets hard to turn it usually indicates your selector is going, aging, it gets spongey. Internally, that selector is very simple, it has a diaphram that erodes over time from the gas. If you remove the side set screw, and slowly wiggle it back and forth, you can actually remove the selector, and you will find a mushy gray diaphram. Unfortunately that diaphram is not replaceable and you have to replace the entire selector. However, to save a lot of work,( removing the lines from the selector) , you could find a good working diaphram,, and swap it with the mushy one.