Any decent meter, including an old analog one with a galvanic movement, can be zeroed out and should be, for leads' and contacts' resistances.
If shorted Ohmmeter indicates form 2 to 120 Ohms depending on light touches to the switch knob and indication changes in time itself that is not the case for trimming/zeroing.
That said, unless someone knows of something better, there's a conductive grease that typically is used to protect lead acid battery terminals from corroding and from acid exposure - I'm guessing that MAY work.
Conductive grease spread over dielectric surface of board between exposed contacts looks irrelevant. I am pretty sure - it is not applicable here.