Go on ebay and buy a small bottle of graphite spray, dismantle the cap give the mechanism a clean and use that to lube the lock. WD40 etc tends to hold dirt and grit and as a paste forms inside the lock it creates excessive wear.
Agree with the others unless you have the bike parked in your own garage when unattended you are asking for trouble - a few years a go a member on here had his fuel line tampered with a couple of times as they were draining the tank at night! Getting it filled up with piss is pretty unlikely but getting the cap pinched is not.