GL-4 vs GL-5

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Al

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A long ago we had a discussion about these specs. In theory most Transmissions call for GL-4 because theoretically the synchronizers can not tolerate the GL-5. At the time this was discussed we had a former rep (I believe-from Mobil) post a differing opinion: He indicated that the GL-4 was no longer a valed spec. He also indicated that the additives in GL-5 have been upgraded to the extent that it is fine for brass synchros. Any updates here ???
 
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I was recently looking at tranny fluids and gear oils, because I want to change both fluids. My '96 A4 Quattro calls for G4 spec tranny fluid and GL5 rear differential oil. GL5 spec lube will attack brass synchros due to high sulfur content. Now, I've seen there is a bunch of GL4 fluids that pass GL5 requirements, while also meeting GL4 specs. Those fluids should be okay to use in applications that require GL4.
 
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