Thank you all. It seems that every question have an answer in this forum. It's great to learn how to maintain the bike

 

K&N website states that you should clean the filter only when the screen is covered with debris. Use K&N's cleaning kit. If you can still see the black metal screen, then it doesn't need cleaning. The oiled filters actually use the gunk as a filter along with the cloth, so you don't want to clean it too often. As for oiling, the element should look slightly red. If it is white or grey, you should lightly oil it with K&N's red oil. The oil is red so you can see whether the element needs oiling or not.

-Beer