Ask anybody with a "gas-brains" about aftermarket air-filters and you'll hear words like "performance", "horsepower", "mileage", "economy". It's a general consensus that aftermarket air-filters like K&N or ACDelco have a distinct advantage over the factory paper-filters when it comes to improving the performance on your vehicle.
Some reviewers suggested that such air-filters don't really give as much of a horsepower increase in naturally-aspirated engines as one might think. Independent reports suggest only a 1-2hp gain in overall power with an aftermarket filter, while other areas such as fuel economy would show greater relative increases. This makes sense in that in order to make an explosion, a certain amount of fuel and oxygen is required. If you have more oxygen than fuel, then you need less fuel to create the same amount of power, hence less overall consumption and better fuel economy.
But when it comes to forced-induction engines, such as our beloved Turbo-charged diesel engines, that's when power-gains become more pronounced. And this isn't hard to fathom, if you apply a little bit of basic physics to the concept.
Simply put, a forced-induction engine would have more air being rammed into the combustion chamber than a naturally aspirated engine. If you have freer-flowing air coming from a less restrictive filter PLUS a turbo-charger compressing and ramming the air into the intake, then you can get a larger amount of oxygen into the combustion chamber.
Because of the higher compression pressures within a diesel engine's chambers, more oxygen mixed with the same amount of fuel per stroke would mean more fuel would be burnt, thus we get a much more powerful detonation and less residual fuel left over. Thus you get more power, and better fuel economy. In a naturally aspirated engine, there is no benefit of extra air being pumped into the combustion chamber.
So how are K&N and similar filters different from paper filters? K&N filters are made out of foam material (similar to surgical gauze, laid out in overlapping layers) that are coated in a special oil. Foam has larger air-spaces than paper. However this oil helps to trap particulate matter within the foam, while still allowing the larger air-spaces to breathe freely. This is how K&N filters are able to achieve up to 50% better air-flow rates while keeping dirt and dust out of the engine.
Besides that, K&N filters have the benefit in that they can be washed and re-used many times over. Though initially expensive (the filters aren't as cheap as paper-filters), this would save you money in the long-run.
K&N boasts that their filters have a "million mile warranty", in that, once they are used and maintained properly, the filter should outlast the life of the engine & vehicle. They also recommend that their filters not be washed more than 25-times, to keep the maximum efficiency of the foam. Also, being that the foam filter is more durable than paper, K&N suggests that the service intervals can thus be longer than with paper filters, i.e. 50,000KM with K&N vs. 10,000KM with paper.
So, what's the final verdict? Should you use an aftermarket performance air-filter?
Why the heck not? 
Here's a few tips to get the best out of a K&N Filter:- Get the CORRECT filter for your vehicle. Some filters are interchangeable, eg. the 3.0L D4D hilux uses the same filter as the Ranger & BT-50
(Part # E2296)- Make sure to follow all installation instructions properly. If the filter doesn't fit or isn't mounted properly, then you lose the filtration effect.
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CLEAN THE FILTER REGULARLY. The recommended intervals for cleanings are between 25,000KM - 50,000KM. These intervals are longer than paper-filters, however you still shouldn't forget to clean your filter. This depends on the vehicle make as well as your regular driving conditions.
- If you're taking the vehicle off-road or drive in dusty areas often, consider cleaning the filter more often than usual. Remember the foam is more porous than paper-filters, so once they're saturated with dirt, the filtration effect is lost and you'll end up fouling the engine.
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Use ONLY K&N cleaning supplies & filter oil, DO NOT replace it with DIY or other brands of solutions
- The best gains in horsepower come from a combination of: K&N FILTER + COLD-AIR INTAKE PIPING. You'll gain more power when sucking in colder, denser air away from the engine instead of hot air close to the engine. Consider some shielding or relocating the filter away from the engine block & exhaust headers (once the engine bay has space).
Here are some links for further reading about K&N Filters:- K&N's official website & FAQ section- A detailed overview of K&N's performance air-filter (*this is an interesting read*)
- A review on the performance of K&N Filters- How to clean your K&N and similar-styled air filter- How to get cleaner air from a K&N Air Filter 
Everything you need to know about K&N filters.
