They come pre-oiled (just Googled), then when you clean it you have to oil it again. Generally, the oil doesn't play too much havoc with the MAF as long as you don't over oil it. I've currently got a K&N panel filter in my air box with no issues. I do want to change it for a dry option though just for ease of cleaning. It could have just been the stage1 map I had but where the initial throttle response was great, getting it to have the power throughout the rev range was poor compared to the standard air box