I replaced Air filter today. It was not as bad as I thought it would be. I just had to be careful about removing fairing pieces. At 4888miles, the original air filter was pretty dirty. 
I have been using my ZX for commute, back road fun and a couple of times to Yosemite but never off-road. I was surprised to see the old filter. My car's filter was very clean after 7500miles. I guess the difference came from how the air is sucked. In the case of ZX, air is sucked from the front side and cars inhale through a small pipe. At least, I don't have to worry about it for the next 5k miles 
1973 Suzuki GT250, 1976 Kawasaki KH500A8, 1979 Yamaha RD350, 1985 Yamaha Maxim700X, 1987 Kawasaki EX500, 1994 Honda CBR600F2, 2004 BMW R1100S, 2005 Kawasaki Z750S, 2003 Suzuki SV1000N
