GL5 contains EP additives that attack yellow metals such as synchronizers. I use synthetic transmission oil on all of my transmissions. It does reduce the synchronizing effort of the synchronizers. But that doesn't concern me. I'm accustomed to "floating" the gears when I shift. Synthetic would be just fine for an old spur type transmission because it doesn't have synchronizers. And the nice thing about synthetic is that you don't have to change it for many years.