Roelof - what exactly is the problem with Mugen filters in your opinion? are they passing too much crap through or just plain disintegrating? I have used them for years now since this is the only INS box that fits nicely under touring bodies and can't really complain, other than that the Mugen filters cost twice as much as anything else.
I've been oiling them with Valvoline motocross air filter oil, really nice stuff.