Whatever you do, don't assume that torquing down the head bolts will 'fix' the issue - you'll only cause more issues when you snap a head bolt.
You may have to take it off and replace it (the gasket).
I don't think overfilling the oil would do this, but is your oil level ok?
If this did this on my bike, it would indicate oil seepage from the airbox, where oil gets thrown if it's too full, but the 919 airbox is not above the valve cover, so that's not it.
The only other question is are you sure it's oil? Not radiator fluid or fork oil?
2009 Aprilia Tuono - Ginger
2001 XR650R BRP (Big Red Pig)
2006 Honda 599 - Ex wrecked it :-D
2007 Honda CB900F (sold)
2006 Honda VTX 1300C (sold)