I went through the same 600 miles and then increasing missed shifts and clutch grab syndrome. Used to get to about a thousand and then just give up and change it. Switching to Motul semi synthetic made it go away. Not only that, but consistent use makes the gearbox and clutch better over time. Full synthetic seems even better.
There are laterally sliding operations in a gearbox and my hunch is that those aspects in this gearbox just perform better with synthetic, wheres the 5 speeds don't seem to notice as much. Maybe bigger parts and larger clearances or less tendency to bind on the shifter forks.
While it's not quite the same gearbox, the six speed came out in the old T20 250 two stroke back in '66. Thus it had its own supply of cooler oil and all the zinc and such that the catalytic converter has made a baddie. The common wisdom is that the gears shear up the VI improvers, but gearbox less motors will do that too, but the tranny and clutch aren't there to complain about it. The other aspect to consider is that the old two stroke had the smooth power output of a four, what with two cylinders firing twice as often and less power, so the 'box had it easier.
I've repeated the same syndrome and fix with quite a number of other's bikes. Sure, I don't like paying big money for oil that's on a one way trip to the recycle, but who wants a grabby and draggy clutch and a cranky gearbox? If you don't have a problem, or don't care or notice, disregard this message. If you do, try a high quality motorcycle synthetic and see if it helps.
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