Has anyone used Amsoil synthetic Transmission lube 75W-90 GL-4 in their 1969 4 speed?

I don't see an attack here, just a statement of some basics. Additionally, what actually failed? Cluster gear, bearings, synchro ring, u-joint, main shaft, or did it totally seize or blow the case?

I am not being mean spirited or judicial here, just looking for more info as to what occurred.


I have run auto tranny fluid and 140w and 90w at different times, no problems, but I think I would go with a synthetic.