sounds like a gear synchronization meshing problem, although I wouldn't say it was a problem if you use the clutch.Some gears are cut differently I supose thats why you find it harder to change from 4 th to 5 th,maybe thats where the drive changes shaft and slides into gear. I would change to a different oil,ie heavier weight and wait for the gears to run in. my opinion

Thanks, looking forward to find out what might be wrong.