There are a number of options available including stock paper elements, K&N and Amsoil oiled foam drop-in filters and the open element filter systems.
K & N Drop-In
The stock paper elements and aftermarket drop-in filters require no modifications to the intake system. The open element sytems generally require removal of the air box, replacement of the intake tube and other potential modifications. I have experience with the open element systems on other vehicles. While there can be performance improvements to be had, these are generally minor and the expense is gas mileage and extra engine noise.
I opted to go with a K&N drop in filter (P/N 33-2139). Mainly due to availability - a local 4x4 shop had one on the shelf. I have used some sort of K&N filter on every car I've owned. There is alot of division on the effectiveness of the foam/oil filters. My experience has always been very positive. I haven't noticed any appreciable dirt in throttle bodies from using these filters. I was hoping to extract a few ponies with this change.
K & N Drop-In Filter for 4.0L I-6 WJ
Cleaning and Reoiling the K&N Filter (from K&N's site)
All K&N Air Filters are washable and reusable. Your normal driving environment will determine when you should clean your air filter. If you drive in a non-dusty wet environment with lots of trees and vegetation, we recommend you inspect your filter every 50,000 miles. If you live in dry or desert conditions, we recommend you inspect your filter every 30,000 miles. Extremely dusty conditions such as driving regularly on dirt roads would require even more frequent inspection. A filter inspection does not necessarily mean it needs to be cleaned. An inspection should be visual to identify the amount of dirt build up on the filter. There will always be dirt and this can even help filtration. Only when the dirt build up becomes excessive should you make the decision to clean the filter. A K&N filter can accumulate dust up to 1/8th of an inch before cleaning is required. The easy way to make this determination is to see if you can visually identify all of the visible aluminum wire mesh on the top of the pleats. If it is still visible, the filter does not yet require cleaning and can be replaced in the vehicle. The next subsequent inspection should occur in 5,000 to 10,000 miles, once again, depending on driving conditions. Once the aluminum wire is covered with dirt and no longer visible in some places on the exterior of the filter, it’s time for cleaning.
When cleaning the filter, first gently tap the filter on the ground or another hard surface to remove any loose dust that will easily fall off the filter. Next, thoroughly saturate the filter medium with K&N’s Air Filter Cleaner. Then let the filter set for approximately 15 minutes. After that, take a garden hose or low pressure water source and from the inside of the filter out, wash off the dirt. Flushing from inside out is the best way to remove the dirt from the medium. Next, shake the water out of the filter and let the filter dry for several hours. A non-pressurized air source such as a common hair dryer, operating without heat, can be used to accelerate the drying time. Pressurized water, air or heated air can reduce the number of cotton microfilaments reducing the filtration ability of the filter in the future and should never be used. When the filter is dry, it needs to be re-oiled using K&N Recharger Oil. We recommend using our Recharger oil that comes in a squeeze bottle because this provides the most control over the amount of oil applied. When using the squeeze bottle, simple apply a bead or line of oil along the top only, running the entire length of each pleat of the air filter. The oil will over a short period of time soak into the rest of the filter medium because cotton is very absorbent. For those of you who want to use our aerosol Recharger oil, lightly “paint” the filter surface with one coat of the aerosol spray. Do not over oil a filter because this can increase restriction undermining the benefits of our High-Flow technology. A filter should never be dripping with oil visibly running off the filter. After oiling, we recommend you gently tap the filter to remove any excess oil and allow the filter to set for a few hours. While this setting time is not a requirement for a properly oiled filter, it will reduce the likelihood of over oiling and is never a bad idea.
Remember, there are a few things you do and do not want to do to a K&N Air Filter because they will cause damage to the product and reduce the service life. Our air filter cleaner has been specially formulated for the specific task of cleaning our cotton air filters. Other cleaners are not recommended and could, depending on the cleaner used, damage the filter. Feel free to use our cleaner to remove grease and oil from your driveway, it works great, but save some for our air filter, which it is designed for. Do not use gasoline or caustic cleaning solutions. Do not use steam cleaning equipment nor car wash devices; pressurized equipment will strip the cotton of those critical microfilaments. When drying the filter after cleaning, your best bet is to let the filter dry naturally. Please do not use compressed air, open flames or heating devices of any kind. If you’re really in a hurry use a hair dryer with no heat. When Recharging the filter with oil, we recommend you use K&N Recharger oil. Once again, it has been developed for our filters and is the same oil applied at the factory during the manufacturing process. Do not use transmission fluid, motor oil, lightweight oil, or diesel fuel.