I have a 03' rancher tm, and I climb steep hills, ride on odd angles, and I have never had oil in the air box. Only oil ever present is from the foam air filter. It does sound like it is coming from the crankcase breather, but i inch sounds like a swimming pool in the air box.

 

Things to keep in mind. The Rancher is a dry sump crankcase, like the rubicon, 400ex, rincon. Like the Rincon, the oil tank is in the crankcase (unlike the rubicon, 400ex). What this means is there is a very specific procedure for checking oil (owner's manual), and if you don't follow it TO THE LETTER, you will probably overfill your machine. The excess will end up in your airbox. Also, you need to check your crankcase breather, from the engine to the airbox. I think there is a small filter on the airbox end. Sometimes, in a cold climate, the vent or filter can freeze, causing the crankcase to pressurize. Again, this will cause the airbox to fill with oil. One more thing, make sure you have the proper viscosity oil. I have found too heavy oil (ie. 10-40 in 0F) will make ranchers prone to spitting oil into the airbox.

 

The foam filter from the crankcase breather (at the airbox) was saturated with oil, so I assume it's coming from there. I run Castrol Motorcycle oil 10W40 all year round. I am thinking that this is from being overfull (from the previous owner), and when I did the oil change I overfilled it just a hare. Before I go out this weekend I am going to drain what's in the airbox and see what happens.