Im new to the 850 Xp, i have an 09 that i picked up recently that didn't have the updated valve cover or vent parts installed. During the process of removing the engine i removed and inspected the air box it did indeed have oil in it.
I have been rebuilding the engine and have purchased a few parts on Ebay. Seeing all the parts available on ebay, and after reading this thread, it got me thinking: there are a bunch of valve covers from 2012 and up on eBay complete with the Labyrinth plate and valve for dirt cheap. I have seen them listed anywhere from $10-$60+. i picked one up from a 2016 with free shipping for $20. I then picked up the vent hose with free shipping for $8. basically i get everything from the "kit" including the valve cover for 1/3 the cost.
Maybe I am missing something, but i don't see any reason why these parts wont fix the oil in the air box problem.
Maybe this idea has already been covered in this thread or others, (there is so many pages that i forget where i read everything)
YES, sounds like a good move. Only concern would be the quality of the used parts, but then that is always a concern. Only thing I would suggest...is maybe getting a new valve cover gasket to prevent leaks. Otherwise...makes sense to purchase from the vast Used parts bins.