One of the members claims he shoots some WD-40 into the lock tumbler part; you may be able to find the thread by doing a WD-40 search. However, I do not know how this works out in cold weather, maybe it could freeze up.
Graphite is the old standby but it will not clean out the lock tumbler or repair a tumbler that is going bad.
However, if you are speaking of the crud/old grease that builds up inside of the actual Switch part you will need to take apart the switch and clean it out and if you want re-grease it. If that can even be done; I have not tried it myself.

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84 300D, 82 Volvo 244Gl Diesel