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

Honestly, You could run the cheapest, low grade gear oil and expect and get more than 500 miles. People do it all the time, they go 10s of thousands of miles on low grade oil. If you lost your trans after only 500 miles my bet is it was not the oil you put in it.