I rounded off the nut on a K&N a couple of years ago, and spent way too much time getting it off after that! Mucho swearing and a bleeding knuckle or two got the filter off. There is a wee swipe of oil on the o-ring out of the box, but I make sure I lube it up real good before installing now. Getting a strap wrench on the filter is all but impossible on a BMW boxer too. On another vehicle I had a similar problem, and ended up tearing the filter off right down to the base, then I used a hammer and punch inserted into the holes around the centre to get the base off.
2013 KTM 350EXC-F, 2005 BMW R200GS, 1984 Kawasaki GPz 750