Some years ago when I was attending many roadside breakdowns around Manchester my employers supplied me with a gallon of Gunk. You decanted it into a spray/ bottle and it was used to spray on everything electrical, as it dispersed and repelled moisture and lubricated at the same time.
It worked well with distributor caps and spark plug leads. Had actually forgotten it existed, but since it has been mentioned it has reminded me of it ... as previously stated .. WD40 is propably the modern equivalent.