I bought a used 2005 yz450 and first thing i did was change the oil, it came out real black and dirty. I didn't have a filter so I left that alone. As soon as I fired it up to check the level the new oil was dirty. So I go and get a filter and some more oil and change it again. It still comes out real dirty right away. how fast does your guys oil turn dirty, I would think it should last longer than this.