the blocking rings, which are the brass colored rings on either end of the synchronizer assembly are notoriously soft on the ax-5 transmission. using gear oil in the transmission makes the problem worse because the high pressure additives in it erode these rings. pennzoil synchromesh is a product made specifically for manual transmissions. i have heard of people using,synthetic oils, atf, or 10-30 motor in these transmissions, but i have never tried them myself.
other possibilities are the three synchronizer keys in the center hub may be worn or broken and jamming the shift collar, i've had that happen before. a bent shift fork is possible. and a main bearing on that transmission shaft is another ugly possibility.
i would expect to have some grinding going on as well as hard shifting with a bad blocking ring. in any case be real gentle with that transmission because if it breaks you don't want to be under the repair bill when it drops. good luck, john