i use a backing block like said above- but when i am trying to get down to that transparent feather on the ends of the riser, i find the up and down movement can damage that fine wood at the ends.
when i worry about the sleeve wearing in one place- i either flip the sleeve- or i have a few pieces of nice smooth birch ply, that i put on the deck of the sander- and my piece can contact a new area on the sleeve.
i had to take my sander apart- and figure out what was causing the oscillation- and then remove those parts-
required me to turn up some metal bushings as spacers to replace the oscillating discs that i removed.
mine had two discs one was static the other rotated-the sleeve was connected to the rotating disc.
and the two discs had hills and valleys on them- so when the hills ran over the other hills- that moved the discs further apart, and the sleeve moved up- and valley to valley the sleeve moved down.
i removed the rotating disc- and put a spacer in there- now shes just beautiful!!