I purchased a new L2501 HST in July. On this forum I learned of a second air filter which may be installed INSIDE of the existing air filter to remove any particles the first filter missed. This filter may not be in the L2501 parts list. The local mechanic told me in my case it is unnecessary - relatively low use tractor. He only recommends the second filter to farmers using the tractor in dusty conditions all day every day. At under $20 I decided it was cheap insurance and purchased one anyhow. They are a no brainer to install. You may put it into the existing filter then replace both at once. You may install it then place the regular filter over it. The air filter cover may be a bit more difficult to replace due to the added pressure against it but if the housing cap is properly aligned with one lip going inside the other it will pop into place. The Kubota part number for the filter is TA040-93110. The description is Element Assembly. Although not included with the L2501 they are the same filter as some of the larger Kubotas use.