I'm running both of my Ducs with STM breather valves with K&N filters. The Monster has had this combo for the last year and a half (about 3k miles) with no problems. I just did the SS this last January and have ridden it about 1k miles with no problems. The reason I changed the breather valves is that the older ones tend to weep oil all over the rear of the engine and both of mine were doing just that. The oil comes from where the plastic top of the breather meets with the aluminum casing. Very messy. I also did not add an extention tube for the filters. Nor did I oil them.

I'm not sure how much of a factor crankcase pressure plays in oiling of the engine since the oil pump is capable of 20+ psi at idle in a hot engine and 60+ psi at 3.5k - 4k in a hot engine but crankcase pressure is maintained by the breather valve and not the tube attached to it. Check out the inside of an STM or Nichols breather valve sometime. You'll see what I mean; little one-way flutter valves. Hope this helps.