We went through quite a cycle in US, trying to find the best transmission oil. And because of DSMs, US had the highest number of 4G63 enthusiasts!
It went something like this:
Redline MTL-> BG Syncroshift or GM Synchromesh-> BG Synchroshift 2 (synthetic version) -> Motul - > cocktails of any of the previous oils...
ALL of these showed signs of excessive wear on cars that were raced! So there doesn't appear to be a perfect oil to protect these transmissions, if you race.
But any of the above also work OK. Once it was discovered that Penzoil made Synchromesh for GM (and sold under its own name, as well as GM) most people started using it because it is cheap and easy to find (at least in US).
It is worth adding that pure gear oils (like Mobil1), might work OK when it comes to protecting the bearings, but they lack friction modifiers, which are needed by the synchros. So such oils should be avoided!