Synthetic GL-5, or dead dinosaurs?
If it's synthetic, you're probably all right - most synthetic GL-5 gear oils don't use sulphur-based EP lubes, and are therefore safe for use with Yellow Metals (copper & copper alloys - sulphur solutions are antagonistic to copper.) Check the label if you used a synthetic.
Dead dinosaurs? Drain & replace at your first convenience. Dead dinosaur GL-5 almost invariably will use sulphur-based EP lubes, and they can literally dissolve the synchroniser rings (sintered bronze) in the AX-15 and other gearboxen that use them. (GL-5 is fine in the Peugeot because there isn't even a speck of bronze in the case. The synchroniser rings are aluminum, the bearings are steel, and I don't recall seeing any bushings in there.)
GL-3 is the preferred lubricant for AX-15 transmissions - when it comes to dead dinosaurs. GL-4 may be used in the short term (relatively low concentration of sulphur,) while GL-5 should be avoided.
As mentioned, most "performance synthetic" gear lubes will not use sulphur-based compounds for EP lubes, so they'll be just fine in the AX-15 (and other gearboxen using bronze synchro rings - which includes nearly all of the better vintage gearboxen. Some newer five- and six-speeds will call for a special synthetic oil - the NV4500 and NV5600 are good examples of this, as is - I believe - the NSG370 six-speed.) Check the label.
Also note that, in a pinch, engine oil may be used in a manual gearbox without ill effect. An old hot-rodder's trick is to drain the 80W-90 out of a Muncie and refill it with 10W-30 engine oil to reduce pumping drag, it can net you a couple of additional horsepower (which can make a difference on the track!) Use 10W-30 - if the MT90 won't be in anytime soon, drain the GL-5 out of the gearbox and replace with 10W-30 until it gets there (any API service spec will work, since engine oils don't use sulphur lubes and the organometallic antiscuff additive concentration isn't an issue for the gearbox.)
The conflict with using GL-5 in the AX-15 comes from the sulphur-based EP lubes and the use of bronze for the synchroniser rings - the sulphur, as I mentioned, is antagonistic to the copper in the bronze alloy, and will literally dissolve the synchroniser rings. However, this doesn't necessarily make the gearbox unusable - just makes it unsynchronised. Gearboxen used in heavy-duty trucks are often unsynchronised, which simply means that truck drivers are taught double-clutching to shift gears smoothly (if you've ever driven a very old manual transmission vehicle, you've probably learned to double-clutch as well.) Double-clutching will be necessary until you can overhaul the gearbox and replace the synchroniser rings. It's usually obvious when the wrong oil was used for the last topoff when you drain - the "gold glitter" you see in the oil is the dissolved bronze synchroniser rings.
Some later gearboxen use carbon fiber or other synthetic/composite materials, which is why a specialty lube is necessary.
Funny thing - recall how I mentioned using engine oil as an old hot-rodder's trick? Several front-wheel-drive gearboxen are spec'd to use engine oil (typically 5W-30, but sometimes 0W-30) from the factory for fuel economy, or even using Dexron automatic transmission fluid (our younger son had a 1995 Neon with a manual gearbox, and it spec'd Dexron III for the fluid change. He thought it was funny, until I explained why they do it. ATF corresponds to a roughly 10-vis engine oil, for reasons of pumping and the like.) Pumping loss/drag is also why modern engines are designed for a much "lighter" oil - with 5W-30, 0W-30, and 0W-20 being fairly typical at this point.
But, dead dinosaur-based GL-5 should be removed at your first opportunity. You won't get it all, but the concentration of sulphur will drop by a couple orders of magnitude with your first change, and will drop to insignificance with the second. Just don't give the sulphur time to work on the bronze, and you should be alright.
