I have read some debates about the effectiveness of the K&N air filters. I don't know if there is any issue with the 850's unless you over oil.

I have been running a K&N box filter for about 55,000 miles (2 years). I went to clean it last month but quickly learned from the instructions that my filter is still not clogged enough to clean and re-oil.

The filter has better air flow when new. When well used, it is suppose to have about the same air flow as paper filters. When they get a certain % dirty, you are suppose to clean and re-oil them. At that point, if you use to much oil, you do run the risk of damaging your MAF.

The maintenance chart suggest that you replace the air filter every 30,000 miles. K&N suggest that you clean/recharge their filter every 50,000.
1993 - 1997 - 10,000 Mile Maintenance Service
The paper filter is $11 while the K&N is $50. So you'd have to change 5 paper filters to pay for a K&N. That's almost 150,000 miles worth of driving. So, if you don't put 150,000 on your K&N it would be a waste (Not considering the recharge system cost of $11, that's another 30,000).

The whole deal with the K&N is suppose to be better air flow and no more cost of paper filters. I believe I got maybe 1 more MPG with the K&N filter. That's when it was new. Not sure if it is any better now that the filter is 60% dirty and has 55,000 miles on it.

When I pulled my K&N filter to recharge it, it appeared to be 30% dirty and had a lot of debris on it. So I cleaned off the large dirt (tap and vacuum) and put it back in because by the K&N definition of dirty and in need of recharge, mine was not there yet. Maybe because I do a lot of highway driving.

 

From reading your comments Robert I feel that you are of an overall similar view to myself with regards to K&N filters. (competition or racing apart)
As I said earlier on baythans thread, I gained no detectable fuel economy benefits but did notice slightly smarter pickup from low rev's and general acceleration especially when overtaking.
I had one installed in my previous vehicle. (Rav4 1996.)
I wish the paper elements went as cheaply here in NZ. They are over $30 NZ here as opposed to your approx' $11 in US.
I will stick to standard filters in myV40 as she seems to be running sweetly with what she has currently and the turbo offers plenty of quick response when / if you need it.