Lubrication technology has come a long way since Fiat spec'ed GL1 for the SOHC transmissions back in the late 1960s. More than 50 years ago. If you are happy with how your transmission works and using 60+ year old oil technology we can stop here. If not...
General perception is that synthetics tend to leak more than petroleum oils. I have heard the complaints that "I replaced my GL1 with MTL and now it leaks". Based on the dozens of units I have been into I would bet that the GL1 was also leaking it just wasn't as obvious. Synthetics flow more readily and therefore _could_ potentially be more prone to leaks but if the case was properly sealed it won't leak regardless of the oil used.
I should note that; a leak is not "damage". Where as MTL will lengthen the life of the various components inside the transmission over GL1, characterizing a leak, assumed to be due to MTL, causing "damage" or "gasket failure" is a miss characterization of the reality of the two fluid's performance. Note: I typically don't use gaskets in the units I build.
Fact: I have personally tested a wide range of gear oils. GL1, various motor oils (both petroleum and synthetic), various gear oils (both petroleum and synthetic). I have tested the name brand gear oils and synchromesh fluids of various weights, even ATF. I have tested these in both street cars and racing applications, and in various transmissions including dozens of X1/9 4 and 5 speeds. I also build other transmissions for specialty applications. Some street and many are performance or racing applications. I have build Toyotas, Hondas, Fords, GMs, BMWs and quite a few Alfas, Ferraris and even a Lamborghini. The Honda units are the only ones that I don't recommend Redline oils for (and there is a specific reason due to materials). I recommend Redline products because they perform better than anything else I have tested. There are others that are nearly as good, I previously noted Castrol and GM Synchromesh Fluid as very good alternates, but none are better. And I don't see problems with leaks on units I build.
I use MTL in my own X1/9s and have for decades. I also use it in both my Toyotas (street car and race car). No leaks from those either.
If you want to get the most life and performance from your X1/9 transmission, use MTL. I have the data to document that statement. Or don't. You asked for advise and I offered mine.