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 ???