Starting with the 3Aj Ténéré in 1989 the gearbox is good for more milage.
The first 3Aj with the black engine has an extra oil supply to the gearbox.
As this line is outside the engine (starts at oil filter towards the crankcase)
it is easy to check.
The next generation of the 3Aj with the silver engine has a higher gearbox rotation speed (primary drive changed from 31/74 to 34/71), but as a result the pressure on the gearbox components is much lower.
I opened an 3Aj engine with 70.000KM, some pitting on the 5th - but still no vibrations before the engine had been taken out.
All regular XT 600 E or K (from 3TB and 3UW to the latest models) are updated to this standard.
Hope this helps