WD40 is a solvent-based lubricant - BEWARE
Headsup on the use of WD40 - it's a petroleum-based solvent that will do a real number on your strings if you use it, and maybe take the paint off your racquet as well. Stinks to high heaven as well.
Silicone spray is silicone spray - an synthetic lubricant that is manufactured without petroleum. There's a huge difference - silicone spray shouldn't hurt your strings or racquets.