[Nickj]

The foam filters are going to be (a bit obviously) open cell foam, just a surface layer of oil won't catch much of the finer dust particles.
IF you are using the filter for short sharp bursts (MX, Enduro, Trials) with a service between uses then you can use a spray application on foam BUT it really isn't good for the more long term use on road bikes.
With multi layer foam filters I always used Putoline Action oil and cleaner, never saw excessive wear in the bore on my comp 2T's despite a lot of use in sandy conditions. The disc pads might just last for one damp day and wheel bearings were junk in a few events but the piston rings were good for half a season and the 'nikaseal' bores just had normal thrust wear... So the filters were working well.