How long will Connors new gearbox last?
 

Simple question, no? But nobody seems to know! Here's the story....
While in Iran (about 12000 miles ago) my bike (1990 XT 600Z with 30,000 miles on the clock at that time) started making a whirring noise, in 5th gear only, at about 3500 revs.
Some very clever people told me that it was likely to be pitting of the 5th gear, and that I should replace it very soon. Well, 10,000 miles later I got round to it, and guess what? They were right. Sure enough both 4th and 5th gears were badly pitted and in need of replacement.
I used Yamaha genuine parts, bought here in Australia.
So my question is, how long will the new gears last? I appreciate that its difficult to tell, and depends on how harshly the bike is treated - but I mean, roughly how far should I expect? My original gearbox had 30,000 miles on the clock when it started playing up. So am I to expect the problem to return in another 30,000 miles? Seems like a pretty serious design flaw, if so.....or was I just unlucky first time around?

30000 miles? so something around 50000 km ?
from my experience this is the minium what a gearbox should last, presumed that the bike isn't treated to bad and everything is mounted in perfect manner.
you should try a 14 teeth sprocket, it helps to ease the life of the gears.
avoid strong acceleration in low revs, especially in upper gears. do not ride in 5 th gear when speed is below 100 km/h (avoid riding below 4000 revs on longer distances).
your bike should have the external oil hose for gearbox greasing and the big oil pump, so from this side there isn't very much you can do.
use motorcycle engine oil; car oils are not designed to grease gearbox as well.
so far,
regards
klaus