If the above information is accurate about the '09 KTM 250SXF filter being the correct filter for '09 Husaberg FE450/570s, then the issue has to be restriction/flow rate through the filter material. Outward dimensions are the same between a HIFLOFILTRO HF155 (old RFS long) and HF655 (newest 250SX-F), however the filter material is definitely different, appearing denser on the HF655. Setting up a very informal flow test by filling the filters from the inside with engine oil, letting the paper soak, and then measuring flow through showed the HF155 (old) passing about 50% more oil. I'm not saying KTM is right to reject stainless filters, just that my results are consistent with their belief that a certain amount of restriction is desirable to prevent cavation at the oil pump. Smells a little like baloney to me, but I don't make the big bucks.