The quickest way to get rid of the residue is with metal polish. I always have a tube of Peek metal polish handy to use on my sewing machines, so I put a small splodge onto the screw.
Then I used some strong crochet cotton to rub the polish back and forth into the grooves of the thread. The muck came off quickly and easily.
Because there was bound to be some more rubbish left inside the machine, I oiled the thread, screwed the knob back in, cleaned that oil off (which came out brownish), and repeated a couple more times until the oil came out clean.
Now the knob is nice and clean, and easily adjustable, so once I get the machine up and running I know I can adjust the stitch.
I am now over halfway through cleaning up this particular machine and should be ready to test the stitch in a couple of days. It is a while since I worked on a machine. I had forgotten how much I enjoyed it!