I myself would use amsoil gl-5 if manufacturer calls for it.as much as i hate paying for amsoil , there are certain situations i find that it is a superior lubricant.especially. where you will never change it again unless you keep the vehicle another 5-10 years.here is another opinion.
GL-5:
So why is GL-5 so bad?
Oils are made from base stocks, refined and then mixed with additives to improve their performance. GL-5's incompatibility is due to the extreme pressure (EP) wear additives. To give the gear oil its EP rating, it's easiest to add a sulpher based additive like MoS2 (molybdenum disulphide) but the sulphur present in the oil (you can smell it) is responsible for eating the copper alloy used to make the synchros.
GL-4 differs from GL-5 by the amount these chemicals are mixed into the oil (1/2 to much less in GL-4). Since the same additives are present in GL-4, they will eat the synchros as well but at a point in time that is beyond the expected service life of the gear stack.
To get the GL-4 rating, the oil has to pass the ASTM D-130 test. This test determines how reactive the sulpher is against a polished copper strip. The strip is also subjected to heating to simulate the running conditions in the gear box.
Many people have put in a call to an oil manufacturer to ask them about the suitability of their GL-5 product for VW transaxles and received an inconclusive response. They may be told it's ok without any details to back it up. Likewise, they may have heard it's ok to use but the manufacturer won't confirm this because they haven't tested and proved it (cost and liability). The tech on the phone may not even know any better and since you are trying to match a modern oil formula to a 30 year old transmission, chances of getting an accurate confirmation is slim especially if his computer system comes up blank concerning your vehicle type.
To obtain GL-5 certification, the manufacturer does not have to pass the ASTM D-130 test. This is the main reason why you should avoid GL-5. So, if you put in a call to a manufacturer about their GL-5, that test result is what you want to verify.
Some people will claim that using GL-5 in a GL-4 spec transmission doesn't apply when the GL-5 is synthetic. This depends on the oil and cannot be generalized. However, it should be noted that many GL-5 synthetic gear oils do not use sulpher in their EP additive package and can pass ASTM D-130.
Even if the manuacturer states the gear lube is "non-corrosive", its viscosity index may be too slippery for the VW style synchros. This means that while the syncros won't corrode, the oil is not designed to grab the hub the way GL-4 does and the transmission will wear out another way.
As a final caution, some GL-5 formulas have been known to permanently coat the synchros in a VW manual transmission reducing its life by 50%.
So choosing a gear oil is both a matter of chemical and mechanical compatibility. In hindsight that seems obvious but the heat of the debate tends to cloud the issue.