Try googling k+n air filter oil on maf sensor, put same words into youtube and get reading.
As a mature boy-racer, lets call me a road-racer, the principle of a K&N filter are right up my streer, however Lynall nailed it by citing the damage filter-oil can do to the MAF sensor. Simply put the old oiled-filter design works great on old carburetted or "flapper" Air Flow Meter set ups, but it has no place on a hotwire / MAF setup. On any modern motor where I do still want a "sports filter", I go for pipercross these days, however, the M47 engine design really doesn't need a sports filter. I'm not just saying that as I'm middleaged now, and could thereforebe perceived as having turned into a fuddy-duddy...
The reason an M47 doesn't need a sports filter is because the purpose of a sports filter is to overcome the inherent flow resistances of a stock airfiler, typically this is done by increasing the surface area of a filter, or by changing the filtration media with a view reducing it's resistance to flow. But if you look at a TD4's air filter, it's already jolly-green-giant scale enormous, like more surface area than the filter on a BMW 3 litre twin turbo diesel, which can be mapped to the guts of 400bhp, or more with supporting mods, on a filter that is smaller, and ergo arguably more restrictive than a freelander's filter.
I'm not meaning to stomp on your enthusiasm, but you'd be better served if you stick to normal filters and take the money saved from not buying the sports filter and, put it towards a "Ronbox" (Synergy 2a), or a better remap, or some of the gucci bits from MuddyMods. Silicone turbo hoses would be my first upgrade, the original rubbers will be old and saffy, and one hose near the front right headlight has a habbit of falling onto a screwhead which rubs through it creating a boost leak.