Hello Andy,

thank you for your reply.

My transmission is a wet one. I've started to disassemble the clutch and transmission, I want to check all possible sources of error.
Currently I'm waiting for the tool to remove the clutch basket, it should arrive at the end of the week.
One thing I already noticed is a missing washer under the clutch basket nut (the one with "OUT SIDE";
No 20 on https://www.cmsnl.com/honda-nsr250r7-1990-l-japan_model49620/partslist/0059.html).
I'm not wondering at all after I've alredy faced some other issues with missing washers or even bolts, I think the pre-owner was overwhelmed with reassambling the engine after tuning tries or better said overhaul.
I will take out the complete transmission when I receive the tool to check wear limits and components etc.

By the way, some guy with another Honda bike in another forum had simular shifting problems, there it was caused by a wrong sided washer, see
http://forums.sohc4.net/index.php/topic,138663.175.html

I'm going to let you know some more details when I've disassembled the transmission. Can I also use 10W40 oil?

Kind regards and thank you very much.
Felix