I can't see anything relevant to applying grease to DMM selector switches in any of those videos.
They keep changing, but...
- The first is a guy changing fuses on a meter
- Is second a comparison of cheap meters
- The third is a guy cleaning muck off the yellow holster of a working Fluke 177 so he can sell it on ebay
- The fourth is a guy slapping excess grease all over the
plastics of a Brymen meter so he can get that "quality potentiometer feel" - knob and case in isolation, no detents or contacts.
- The fifth is a meter comparison, where he
does smear some vaseline on a spade connector when measuring its resistance (2 wire)
- The Last is using anti-corrosion / dielectric grease to protect crimps and connectors in an industrial / automotive environment.
Don't waste your time, we're being trolled!
EDIT: I think the links have stabilized now!