WARNING!
Do not EVER put a GL-5 oil in a manual transmission UNLESS it is specifically placarded for it by the manufacturer, and essentially NO gearboxes are.
You will DESTROY the synchros.
The reason for this is that GL-5 EP additives (the chemical package in the oil that resists gear wear) works, as do all such packages, by depositing on the surfaces in the unit. The film strength is weaker than the metal and thus shears first, protecting the metal.
The problem is that GL-5's package is STRONGER than the shear strength of the synchros! As a result the synchros will shear (wear) instead of the deposited film on them which is exactly the opposite of what you want to happen.
The (relative) dearth of MTX boxes over the last couple of decades has made GL-4 lubricants harder to find. If the label has BOTH specs on it then you still CANNOT use it; you must have a lubricant for a MTX in nearly all cases that does NOT have the GL-5 certification anywhere on the label.
Redline makes a suitable oil (MTL) and then of course there are the "official" VW oils as well. Synchromesh is a potential choice but it is IMHO not the best option; it is a very light oil comparatively (viscosity wise); I prefer the Redline MTL as it has better physical properties. It can be somewhat difficult to find locally but there's always getting it off the Internet.
GL-5 full-synthetics (e.g. Mobil 1) are great for differentials OTHER THAN THE OLDSTYLE POSI-TRACKS which have the same basic issue as does an MTX. The "roller locking" style ones found in trucks, for example, are fine with a GL-5 and in fact it's considered "required" in many applications.
But never, ever put an oil with GL-5 on the label in a MTX without a SPECIFIC recommendation to do so by the manufacturer unless you wish to tear it apart and replace the synchros at some point in the future.
VW's "official" oil is G070, which if I understand things correctly is quite light in viscosity, but it's what I've been using in my '03 without complaints. I have Redline MTL in my Mazda and like it, other than being rather notchy in VERY cold conditions (e.g. below zero outdoors overnight) until things warm up a bit.
Did some research a while back and pulled the viscosities of a number of fluids as pulled from various threads and manufacturer sites:
CsT is centistokes, and is a measure of fluid thickness (viscosity), measured at 40 degrees C and 100 degrees C
--------------CsT@40 deg c-- CsT@100deg c
VW G70-------------- 31-------- 6.2
Fuchs Titan sintofluid- 41-------- 6.7
Penz(GM) syncromesh 41.6 -------9
Amsoil Syncromesh ---48.3 -------9.7
Redline MTL 70W80 ---56.2 ------10.6
Redline 75W85 -------64.3------- 12
Motul: 5.2 75W90 ----72.6 -------15.2
Mobil1: 75W90 -------99 ---------15.2
Redline MT-90:75W90 106 --------15.6
R. Purple: 75W90 ----119 --------17.5
Lucas: 75W90 ------ 142 --------18.4
Odd that you're concerned about Syncromesh being too thin yet running a fluid far thinner?