You maybe have a 2012 car Swedewheeler but it looks like you have a later oil tank as that cap is from a later type. The early tank has a dipstick in the cap, as well as an O ring seal. Many early tanks have been replaced as they had oil leaks from the end plate welds.
If you fit a new seal made from some oil resistant rubber sheet and retained with a large penny washer, as in the photo in my earlier post, that will fix this problem. All caps should have been modified by now but that relies on the owner either doing it or taking it to a dealer that knows what they are supposed to do.
