[GTJIM]

Welcome to the OpelGT Community!

Check with your local BMW agency or a BMW manual at your local library!
They use a very similar Getrag 240/245 5-speed trans.

Just found this:

"Oil should be seen as an integral part of the transmission. When designing the transmission the load bearing capability of the oil is part of the calculation. We fill the BMW transmissions with SAE 90 GL 4 gear oil manufactured by Fuchs, a brand mainly found as an OEM supplier.
SAE 90-Oil should be used throughout the whole year. It is true that in winter the shifting will suffer at first from the thick oil, but it should improve during a very short ride. For those to whom this is disturbing, because they make many short trips, for example, can use 75w90 GL 4 in winter as an alternative. In summer it must be changed back to SAE 90 GL 4."

Also this:

Read the label:
In order to choose a gear oil, you have to CAREFULLY read the label. GL-4 is the recommended API rating (American Petroleum Institute) and SAE 80W90 is the correct weight to use. Sometimes GL-4 oils are identified by these other classifications:

MIL-L-2105 (US military spec)
501.50 (Volkswagen)
235.1 (Mercedes Benz)
Note that MIL-L-2105D and MIL-L-2105E, etc are not the same as MIL-L-2105.

If the label makes no mention of GL-4 then you've probably picked the wrong type of gear oil.


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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 Getrag 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 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.