The volume control on a IMA303 will become stiff and difficult to turn, cleaning with DeoxIt D5 will solve this problem. The selector switch should not be cleaned unless it will not move at all.
Becuase of the long slide lever onthis switch, it tends to become loose internally and cleaning will often make it impossble to slide.
This swicth is not avaialble form NuTone but it can be sourced from other vendors