I've noticed little pockets at the front of the shuttle travel with felt in them on several of my machines. In the wall from the pocket to the shuttle is a small hole. Those are to be filled with oil so the shuttle doesn't ride dry on the machine.
I've removed the old decayed stuff in the pockets, cleaned them and the horizontal hole and put in new felt. Then each time I use my machines I fill the pocket with oil.
That actually made a noticeable difference in how a couple of them sounded. It can't do anything but help the machine.
For the machines that do not have the oil pocket I put a drop of oil on the arched wall of the machine every time I put in a new bobbin.
Joe