I find that if I change my oil every few rides I do not run into any problems. My steel plates tend to turn my oil greyish. But, aside from that. I'd change it every month or so depending on how you ride. I ride the hell out of my KX and have to maintain it constantly to keep it in top shape. I've changed my oil so many times that I've had to put in Helix Threads at least 10 times in the past year.