Gears won't shift.

No that you mention it, it's mostly up-shift. 90 % of the missing shifts is up.
You think this could be an adjustment issue?

I know on mine. as the years went by, every time it wouldn't up shift nicely was because the clutch needed adjustment. I suspect it wouldn't disengage completely.
It only takes a minute to adjust it from the handle. Easy thing to rule out.

Change the oil, 5w40 is very light, that will definitely change the feel of your clutch. I use 10w40 only because of the cold climate here

Does it ever pop out of gear?
 

Hmm, since Denmark can be a bit cold I thought a 5/40 would be good, especially for the cold starts.
Next time I'll put on 10/40 and see if there are any difference.
1st gear is very clunky, no matter the speed.

Thankks for the reply, I hope a good new oil will help.
 

Hmm, sounds like a possible solution, I'll try 10/40 and try to adjust the clutch.
It never pops out of gears.

Thanks