only ax-15 i have seen fail not from driver/neglect but from to much HP, was a supercharged v8 cherokee at the drag strip (which was wayyy above the HP rating of the transmission) made a big noise while shifting and it locked into 4th gear.
when i took it apart the 3/4 gear syncronizer hub was detroyed
the other ax-15s have been from people grinding gears and destroying the synchro's
deteriorated fluid (contaminated with sludge, which fills the grooves of synchros and causes them to fail)
not changing fluid causing bearing failure
ive also seen people reseal the input shaft and put way to much rtv on the machined surfaces and block off some oil galleries to in input bearing.
the best was the reverse to 5th clutch drop that sheared 5th gear right off. (i had to replace the main shaft and synchros for that guy)
any used transmission is a gamble, you don't know how the previous owners have treated it,
pull the drain plug and inspect what comes out before you make your purchase (look for bearing material/sychro bits)
from my experience running 10w30 (i have been using 10w30 in various ax-15 for 8 years, i use castrol GTX) has been the best fluid to use.
it literally cleans all the grooves out the synchros giving it a better friction coefficient to allow for a smoother shift.
first oil change after a few 1000K and you won't believe all the sludge that comes out, put castrol GTX in a high milleage (over 400,000km) transmission ran it for 5000 km (it instantly fixed the shifting btw) and drained it, took it apart (converting to external so why not while i had it out) and it was immaculate inside. cleaner than any of the lower milleage running gear oil, it made the synchros look brand new.