I have two (2) MFJ roller inductor based tuners and cleaned both rollers and roller shafts with both rubbing alcohol and spray contact clearner with paper towels sitting under the roller inductor assembly.  This totally solved the "scratchy" rx and intermittent high SWR issues.  If I were to use any type of lubricant on the roller-to-shaft it would be the high priced stuff which contains lithium grease infused with silver (~60%) which has an extremely low electrical resistance even at RF.  IT IS EXPENSIVE so won't be getting any unless I need to revisit the issue soon.