Yep I agree the K&N filters are so much easier to remove with the nut on top... all I use nowadays as same filter on my SV1000...K&N make a good replacement for the 138 I believe as most Suzukis use the same oil filter, also it has a 17mm hex on the filter which makes fit up & removal a cinch with a ratchet & extension.
Gsxr 1000 oil filter is the same part. Latest few models use the same oil filter. A lot of Suzuki models use the same filter.
Yes! That happened to me too on my other bike. I finally got the k&n filter off by removing the exhaust to gain better access and using vice grips on what was left of the nut. On that bike next time I will slightly hammer the socket on before attempting to ratchet off the filter.Those bolts are useless, here's what happens if the filter is too tight: