I have used the Yamaha filter, 5D3-13440-09, the whole time I've owned my WR450, and am about to order a few more. Boats.net is normally my go-to, but I just found Yamahapartspro.com, and they are $.64 cheaper per filter. While I don't really care about 64 cents, I do care about 10% cheaper, and it works out to $6.92 a filter. Anybody find them cheaper? One thing I'll really consider before this purchase is buying the Scotts filter. It's $69.95, 10 oil changes at the price above, which I will probably do before the end of the year on that bike. Added bonus is it carries forward to what will definitely be my next bike, the YZ450FX. I probably just derailed my own thread, but curious to see where most people get their oil filters.