The Redline has been pretty well documented to work in the X1/9 transmission and is a good choice. If you can get the added protection of a synthetic GL-4 and it isn't detrimental to the shift quality then why not? It's not very expensive and easily to get in this country.
That is my opinion which I'm entitled to just because everyone with a keyboard is entitled to one.
I also think I'm qualified to chime-in on the science of the subject having spent quite a few years (29 to be exact) on the engineering side of the automotive industry with a particular focus on designing drive-line components. Not trying to ruffle feathers, just laying it out there for your reference...
Redline MTL is a synthetic GL-4. GL-4s do generally have some amount of EP additives in them, though probably less than half what a GL-5 would have. GL-4 was designed for spiral bevels and GL-5 for hypoids, which are spiral bevel gears in which the centerlines of the mating gears do not intersect.* There is no classification that is older than the other. GL-1, GL-4 and GL-5 are all the same age so you can't really credibly state that GL-4 is modern and GL-1 is old. It wasn't like GL-4 obsoleted GL-1. They were designed for different things. There is synthetic GL-1 (if you look hard enough) but it still won't have EP additives in it so it will rely solely on its ability to build a lube film whereas the oils with EP additives form a semi-solid like layer to prevent metal-to-metal contact during extreme pressure events....which is also a mechanism that can screw-up the effectiveness of synchronizers in some cases and lead to clashing/damage/etc.
The
only GL classification that expressly forbids all EP additives (and friction modifiers) is GL-1.
GL-1 has fallen into disuse by manufacturers mainly because gearboxes have gotten more power dense and pressures have increased for both bearings and teeth. By modern transmission standards, GL-1 is simply inadequate for a the job basically because they need the EP additives now even in non-hypoid arrangements, but that doesn't mean it isn't adequate for a transmission that was designed to use it.
From the www:
GL-1
Status: Inactive
The designation API GL-1 denotes lubricants intended for manual transmissions operating under such mild conditions that straight petroleum or refined petroleum oil may be used satisfactorily. Oxidation and rust inhibitors, defoamers, and pour depressants may be added to improve the characteristics of these lubricants. Friction modifiers and extreme pressure additives shall not be used.
GL-4
Status: Active
The designation API GL-4 denotes lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid (see note) gears operating under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications where MT-1 lubricants are unsuitable. The manufacturer's specific lubricant quality recommendations should be followed.
*Just for fun, GL-2, which is no longer active, was for worm gears.
Source:
http://www.api.org/~/media/Files/Ce...blications/1560-Eighth-Edition-April-2013.pdf
http://www.machinerylubrication.com/Read/28470/role-of-extreme-pressure-additives-in-gear-oil-
https://www.redlineoil.com/mtl-75w80-gl-4-gear-oil
http://www.machinerylubrication.com/Read/30366/gear-oil-applications