The standard fill for MT gear oil in VW's has been VW G50/G51 GL-4 which is 15.6 cSt (probably a 75W-90).
Many of the newer fills for MT's (~ 2004 on) are either …
VW G-052-171-A2
VW G-055-726-A2
… which are ~ 6.4 cSt gear oils …!!!.
These figures are from some VOA's they ran at tdiclub.com. In fact, even for some older (pre -2004) vehicles, if the MT was making a whining sound (under certain conditions) they recommended changing the MT fluid to the newer 052/055 fluids. There is a long thread about this at …
tdiclub
Here are some VOA's they ran on various gear oils …
(Manual Tranny Fluids)
16.7 = Motul MOTYLGEAR 75-90 GL-4/-5
15.6 = VW G50/G51 GL-4
15.6 = Redline MT-90 75-90 GL-4
15.2 = Mobil 1 Synthetic 75W-90 GL-5
15.2 = Motul Gear 300 75-90 GL-4/-5
15.0 = Elf Tranself Synthese FE 75-90 GL-4/-5
14.9 = AMSOIL AGL 80W-90 GL-5
13.8 = Amsoil MTG 75-90 GL-4
10.6 = Redline MTL 70-80 GL-4
9.6 = AMSOIL MTF Synchromesh Trans fluid (GM/Chrysler) None
9.08 = Penzoil Synchromesh trans fluid None
7.5 = Redline D4 ATF Dexron III / Mercon / API GL-4
6.5 = VW G-052-171-A2 None
6.38 = VW G-055-726-A2 None
6.3 = VW G52 (part numbers G052726A2 / G05272601) None
(Automatic Tranny Fluids - except for Redline D4 dual-use)
8.3 = Honda CVT Fluid
7.6 = Mobil 1 Synthetic ATF
7.5 = AMSOIL Automatic transmission fluid ATF
7.5 = Redline D4 ATF Dexron III / Mercon / API GL-4
7.4 = Mobil 1 Synthetic Dexron/Mercon
7.2 = Redline Synthetic ATF Dexron II / Mercon
7.1 = Mobil 1 Synthetic Multi-vehicle ATF
7.1 = AMSOIL Ford type F auto trans fluid
5.5-6 = Ford Mercon SP
Over on some of the VW forums there are a fair amount of people using Synchromesh fluids like Pennzoil (~ 9.08 cSt) in their MT's that specified 75W-90 or 80W-90 gear oils -- with good results -- mainly to improve shifting quality (particularly in cold weather).
I think the 75W-90 GL-4 gear oils tend to be too thick for light duty MT's (cars), and are more suited for rear differentials or more severe duty transmissions in trucks. Given that VW is now using a 6.4 cSt MT gear oil -- and has even back spe'cd it, I think a good case can be made that one shouldn't have a problem with MT longevity with a fluid like Synchromesh. Although, I would be probably be inclined to change the fluid out every 50k miles or so.
My 1987 Jetta specifies 80W-90 GL-4 fluid for the 5-spd MT. These heavier viscosity MT fluids (~ 15.0 cSt) not only make for lousy shift quality, they are a total joke in the cold weather here (hits -30C or lower in the dead of winter). Even tried a synthetic 75W-90, and it still felt very thick in the winter.
There were a fair amount of cars in the 1980 -90's that specified 10W-30 motor oil in the MT's, and they seemed to last without issues as long as the oil was changed at reasonable intervals. Last summer I put in a 60/40 mix of (5W-30 motor oil/ 75W-90 GL-4/5) -- couldn't find any Synchromesh. My thinking was that I wanted some EP additives in the mix rather than straight 5W-30 motor oil -- but I wanted soemthing thinner and with better cold weather performance. My mix would be a little higher viscosity than Synchromesh, but it has improved my shift quality tremendously compared to the previous 75W-90 oils I was using. And this winter in -25C weather, it performed very well.