Why does it say in the owner's manual not to drain the crankcase? I've been changing my own oil for years but only in my vehicles, never on a motorcycle. My bike is a 2008 Pitbull, and I am changing the fluids for the first time, maybe ever. The bike only has about 3k miles on it, and I'm not 100% sure the PO ever changed the oil. It is draining as I write this, extremely black and thick. Also, the oil was low, maybe down 1/2 quart. Any advice would be greatly appreciated!