It was happening because the labyrinth plate that is supposed to stop oil from going up the breather line and into the air box was plastic and would warp if put in incorrectly, which it was most of time. I had mine fixed and I still would get oil in the air box. The new plate is metal and can't warp. They also changed the breath and hose size going up to the air box along with some O-rings. Lots of people had this problem check this link out: Polaris Tech Tip: Oil in Airbox Fix - Polaris ATV Forum

 

I'm going to have to check my 2010 850XP again, last time it just had oil residue in the bottom, not much...

Jim

 

My 2011 850 XP has had oil in the airbox twice now. Took it to dealer the first time, but after about a month, problem is back. Tried the fix in the above link, will keep you posted as to how it turns out. I have heard and read that the solution is to install the 2012 parts as the problem has been rectified in the 2012 models. As to what parts to replace, I am not sure.

 

I did not know about this problem when I bought my 2011 850, This issue sparked my intrest early on when I read about all the problems so I've been checking in regularly, 3.5 months now and still bone dry, tonight I took a look in my 2011 owners manual and the air box does not look the same as what I have in mine, I searched for a 2012 diagram and it looks the same as what I have, it was built late June/11 maybe I already got the 2012 parts in mine dunno

 

A buddy of mine also has a 2011 850XP that is still under warranty and he called his dealer and they are going to ship him the kit of parts for the 2012 upgrade. I will post the parts that are sent to him when he receives them.