On the 800s there are really only two reasons for oil in the airbox other then being layed on it's side or top. The first is the prefilter bag they put over the intake tube has become packed with dirt and dust which creates a higher level of vacuum on the box..which pulls more vapors and some oil through the crankcase vent tube. The next is excessive blow-by. This is leakage of compression and combustion by the rings...and valve seals, pushing the outflow out that same tube to the box. If you have been running it with the stock paper airfilter, then expect the top end to wear out like this very quickly. This is the very reason we always strongly suggest it be replaced before any riding. A fast way to check the blow by is just run it with the airbox cover off and note the out-flow of gasses through the tube. An engine in good shape will just pulse mostly through the tube. One that has an issue will build pressure but don't hold your finger on it to build as that will push-out seals...not a good thing.
There are other things that can show some oil like an over-oiled foam filter. It can work down and drip on the bottom. An over-oiled foam filter will also cause an oil mist be drawn into the intake and coat the air temp sensor. That can easily be cleaned with special cleaner though.