I feel it's necessary to point out, after reading some questions and misinformation that...
Pure silicone does not attract dirt
WD40 is NOT silicone. While the ingredients are proprietary, it is in fact an oil based mixture. (On a side note, don't use WD on your sliding glass door trac because it attracts dirt.)
Lubricating your strings prolongs the life of your stringjob and increases spin potential by reducing the notch-causing inter-string friction, thus allowing strings to move back and fourth more freely.
Lubing your old, notched strings, may give back some spin potential... But you won't get nearly the same effect as doing so to new string.
It has been pointed out by the various tournament stringers here that, they have not applied or witnessed pros applying lube to their strings. Their racquets are bagged because it adds to a more professional string job as opposed to just throwing un-bagged racquets into someone's bag.
There's no good reason to feel bad about getting lube on someone's balls. It doesn't hurt the ball, nor does it cause the ball to pick up a significant amount of dirt in the short time you're going to use them. If you or your opponent aren't discarding or throwing them into your practice basket after 2-3 sets, you should be.
PS85, 367g/345sw, Pacific classic@58