I used a drop-in K&N filter in my stock airbox before I installed my WeaponR setup. I was pleased with the K&N. Its major advantage being supposedly higher airflow rate and is intended to be cleaned and re-used as opposed to being replaced when its gets dirty. They have a special cleaning kit that costs a few extra $$, but the fact that your not tossing airfilters out when they simply get dirty might save a few bux in the long run.