Most manual trans require some sort of friction modifier for the Synchros to engage properly.
Gear Oil, conventional or synthetic, does NOT have friction modifiers and that can cause problems with the Synchros, if it isn't shifting problems it could be wear. Gear Lube is NOT for Limited Slip Differentials either, you either have to add friction modifier or get a fluid specifically for LSD. Not sure on AX-5/AX-15 seen different opinions, but I figure, get a good fluid that has friction modifier in it, it can't hurt to have a fluid formulated to work with Synchros instead of a Fluid formulate for GEARS ONLY. There are plenty of manual trans out there, that if you use a Gear/Motor Oil without any Friction Modifiers, you'll have all sorts of shifting problems, if NOT damage the Syncrhos. But I haven't tried it in an AX-5, so I don't know if its one of them.
Do a search, there are several threads discussing all of this, as well listing suggestions as to good fluids to use.
Another point, there is some confusion on the fluid for the AX-5/AX-15 and GL-5 being recommended, IT IS NOT, it was mistakenly put in some of the owner manuals and pubs about the vehicle. The AX-5/AX-15 recommended is a GL-3 based fluid. GL-5 has additives that can be corrosive to the synchros if it gets hot enough. Same with GL-4 to a lesser degree.
I have used Redline MTL in other vehicles with great results, seen lots of recommendations to use Redline MT90 (a slightly thicker version of MTL) on this board. Both have the Friction Modifiers for Synchros that I was talking about. This is a GL-4 Fluid, but the Redline Literature says that it will NOT become corrosive to brass/bronze Synchros until it gets to something insane like 450°. Others have given Redline MT90 the thumbs up on this board as well.
I have 4 quarts of Redline MT90 in my garage, waiting to be swapped into my AX-5, I'll let everyone know the results.