The debate has been raging for a while now: what kind of air filter is best for off road use in your four wheel drive vehicle? It's not an easy question to answer, as the two types of air filters most commonly used--paper and oiled cotton/foam--have strengths and potential weaknesses.
Paper-based Air Filter
Given that no 4WD manufacturer sells a new 4x4 vehicle with anything other than a paper-based air filter, this speaks volumes about the filtration qualities of paper air filters in off road vehicles. However, almost all off-road motorcycles and ATVs use foam or cotton-fibre oiled filters. Why the discrepancy? Heavy 4x4s aren't usually performance oriented, so sacrificing a few horsepower for better air filtration is an easy choice. Motorcycles and ATVs usually need all the power their smaller engines generate, and oiled filters provide better airflow than paper.
Oiled Cotton Membrane Air Filter
Why do oiled filters provide better airflow than paper air filters? Simply because there are more and larger holes for the air to flow through. Instead, these air filters use oil to capture particles while still allowing the air to flow freely. Most, if not all, performance air filters are oiled air filters.
Which Air Filter is better for off road use?
The answer to this question seems to depend on who you ask. For many, the sheer size and filtration capabilities of 4x4 manufacturers stock paper filters means they would never consider using an after-market oil-based filter off road. For others, the fact that virtually all off-road motorcycles and ATVs use oiled foam or cotton fibre filters means they are suitable for heavy off road use.
Several companies have performed side-by-side air filter comparison tests, showing the filtration and airflow results of their particular brand of off road air filter (here's one example, here's another). However, the results are often inconclusive or unsurprisingly support one brand over all others.
There are obvious performance and fuel economy advantages to hi-flow air filters. On road, it's hard to argue against them. Off road it's not so clear and for many, paper is the only air filter choice for a 4WD truck that will see off road use. But it should be remembered that oiled air filters are reusable and washable, whereas most paper air filters are not (they can be blown or vacuumed, but never really washed).
The Verdict: Make your own call
The choice of which air filter is best for off road use is yours to make. Paper air filters provide better filtration but poorer air flow, and are generally not washable or reusable. Oiled filters provide much better air flow and are washable and reusable, but may not filter as well as paper (this may be perception however).
Run an oiled filter off road and on. The airflow, performance, and fuel economy improvements are worth it, and the cost savings of cleaning and reusing the same filter will pay for a few extra tanks of fuel. If you're really worried about micro particles entering your engine, employ an old trick used by motocrossers: apply a light film of oil or grease to the inside of your air box. This will trap most of the particles that enter the air box before they even get to the air filter.