Don't remember seeing this posted before. Had to drain my tank to send my HPU to Power Pole for some maintenance, below is a pic of the egg sized piece of sludge that came out of the tank. My Sportsman II manual says that there is no stated change interval for the fluid but only to change if cloudy or milky. My 5 year old fluid wasn't cloudy, it was almost black. Probably checking yearly would be a good idea. BTW, great service from the folks at Power Pole!