For those of you who have opened your air intake box, you've probably noticed that it had some oil drip/pour out. I have seen anywhere from a teaspoon to 1/2 cup of oil in air boxes. This is normal. It's just oil vapors from the crankcase ventilation system. The crankcase is connected to the air-box/breather so the fumes get re burnt and not vented to the air. Some oil will collect in the cover and not stay a vapor.
If you want to help your air filter last longer, and if you don't want the oil vapors to be re burnt, you can add a crankcase breather.
On the 900 it is very simple. Remove the coil cover on the left side. Disconnect the tubing from the back of the air box and Rotate it, keeping it attached to the crank case (approx 90 degrees). Attach a breather--you can either leave it straight up, supported by the tubing, or mount it to the bike like I have.
Crankcase breathers can be purchased online or at any auto parts store. They cost around $8-$15.