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

Yes, I have been using it for quite some time. My trans is a smooth-shifting pleasure. I also use nothing but their 20W-50 Severe Service Racing oil in my 383. The first thing I noticed there was that the engine turns over faster when cold. I guess their claims of reduced friction are valid.