I was only speaking from what I have heard/read. I have had no issues with synthetic lubricants either.
The difference between GL-4 and GL-5 is the amount of phosphorus in the formula. GL-4 has about 1/2 the phosphorus as GL-5. In a typical rear axle with steel gears, the phosphorus forms a black sacrificial layer between the gears to provide protection. That's a good thing and the steel is harder than the phosphorus so it peels right off afterward. In modern manual transmissions, with brass synchros, the phosphorus forms the same black sacrificial layer but the bad is that its HARDER than the brass, so each time the sacrificial layer is peeled away it takes a micron or two of the brass along with it. Over time you can guess what happens.
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