Grease is more easily pinched through than corrosion. It extends the
life of sliding switches by reducing friction.
Kevin Mc

And by extension, oil is better at preventing corrosion while not sticking up
things.

This research found that simple mineral oil reduced contact resistance
dramatically:

http://www.te.com/documentation/whit...df/p154-74.pdf

I (op) decided Im going to cleans out this mode switch and try Caig
DeOxit.

Thanks for all the great discussion.