I like HPI's 2-stage filters: https://www.hpiracing.com/en/part/87207



The inner foam is oiled (and appears to be made of a material that doesn't degrade from oiling, like a lot of older air filter foams do), and the outer foam is dry so it can be dusted off easily. I use them in combination with Associated Factory Team air filter oil, which gets sticky after being applied. The foam has a lot of surface area so it doesn't restrict airflow. I use them on almost all of my engines, even a 6.5cc 4-stroke engine.