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What I did was to remove the gauge cluster and remove the speedo from it, take some electronic cleaner and spray the shaft bushings where the cable hooks on to soften up the old oil , It is kinda self explanatory when its in your hand on what to do. Then lube it with some light lubricant, wd 40 is not a good lube for this, tri-flow or some good teflon based stuff works well.
This is not a guaranteed fix but it worked for me, I had tons of dust stuck on all the oily spots and it made for erratic movement.
As far as the cable goes I take it out of the casing and use some thinned down never seez or lubriplate no.105 motor assembly lube. Too heavy of grease and it will rip the plastic gears to shreds.
DONT use brakleen on the speedo, removes numbers and such, no it wasnt me but I did laugh ,the guy was one of those know it all jerks, and I quote"it'll be just fine". So I let him spray away