If it is audibly 'scratchy' through an amplifier then inside the pot wants spraying with servisol switch cleaner, as for lubricating the mechanism i would clean it up then use WD40 over oil/grease because they will attract even more crap.
There is a silicone lubricant spray but i'm not sure were they stock it, that is the best for exposed mechanisms because dirt, dust, etc. wont stick to it.