I ordered a K&N from RN way back in 02 for my 91 Disco V8. It too made a huge improvment in MPG and HP. Plus taking the horn off made the incoming air make a lot more noise and sounded cool!
Best bet is to use the K&N for every day driving and change it out with a paper filter for dusty trails.
Or even better get a precleaner wrap for it, if it will fit in the air box.
As stated before a snorkle will help to get it up out of the dust, maybe even a 2nd filter on top to pre-filter it.
I dont have to worry too much about dust here, but in Uganda 90% of the roads were muram (packed red clay) and got quite dusty in the dry season. I checked my filter often but didnt see any caking of red mud on it or any in the air tube either. Hopefully for the guy who bought it off me the cylinders didnt get scored too badly if dust did get by.
First but gone: 91 3 door Disco "White Rhino"
77 Series III 88 ex MoD "Shongololo"
Gone and I miss her: 97 D1 5 speed
04 DII
08 D3 (LR3)