Some GL5 are too slippery and don't have the right coefficient to grip the syncros and so are best suited to diffs .
The yellow metal thing is from the 1970's . GL5 EP additives contain more sulphur but not too much...
"Containing some sulphur isn't an issue, it's the level in the oil that can be a problem and depends on the overall additive package. In the additive package there will be additives to control the acidity of the oil, if the sulphur compounds do produce an acidic byproduct (based on them breaking down, they might not break down), the buffers should control the acidity of the oil. If the additive package doesn't contain enough buffer, the acidity of the oil might be a problem. In reality, the Fuchs and Castrol do not damage yellow metals, we have sold plenty of both of them to know there isn't an issue and companies like Fuchs and Castrol would not be recommending them for use with synchros if there was a problem.
The problem with additives and yellow metals comes from the 1970s and was soon rectified by reputable oil companies. The problem was that original GL5 oils contained levels of sulphur that were too high for yellow metals, leading to corrosion, but when the problems were discovered, the additive package (used by reputable companies) was altered so that the level of sulphur was lowered to a safe level (basically a higher dose of a GL4 additive pack was used)"
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