Its probably oil & water vapour condensate from the crankcase breather system, just needs cleaning out occasionally. Happens more if you do short journeys esp in this cold weather. There is no oil supply to the airbox. The other possibility is that you have a non standard oil filter, the type that has to be oil soaked and some XS has dripped out. I'm sure its nothing serious there's just nothing oily or watery around there, I just double checked on mine.