Hmmm - Had Isuzus in our fleet since the mid 80s and in a big way since the 90s, never heard of one gearbox change.
Make sure you check your mirrors before and after every turn and regularly if driving along a road (say every 3rd/4th pole). Make sure you dont speed - keep to the last sign you saw even if you can see (say) the 80k sign up ahead.
Martyn

