Won't be the oil
I suspect you've either got an issue with the slipper clutch, maybe the spring is fitted wrong or the plates have warped or you have some quality issue with the machining of the gearbox parts. May be swarf in the selector drum. The wrong oil would cause clutch slip. The fact it changes fine on the move with or without the clutch makes me think its a clutch issue but as I haven't got a micro of the gearbox I can't tell how the slipper clutch works so can only guess at the issue.
I suspect you've either got an issue with the slipper clutch, maybe the spring is fitted wrong or the plates have warped or you have some quality issue with the machining of the gearbox parts. May be swarf in the selector drum. The wrong oil would cause clutch slip. The fact it changes fine on the move with or without the clutch makes me think its a clutch issue but as I haven't got a micro of the gearbox I can't tell how the slipper clutch works so can only guess at the issue.


either long legs is a wannabe drag racer or something else is going on. Lack of oil would cause this sort of damage, but that would men that at first service they forgot to put enough oil in