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

It is highly unlikely the gear oil had any direct bearing (no pun intended!) on this particular failure, especially considering the 500mi. interval mentioned. Lube-related failures are more long-term issues, generally speaking.

However, I believe a couple of points WRT the subject of Amsoil's so-called "warranty" deserve a look. 1) Does anyone know of or ever heard of Amsoil paying out a warranty claim? Of course not. They never did and they never will. 2) Their so-called "warranty" is a major element of the marketing machine and not worth the paper it is printed on. It cannot be substantiated in any court of law, based on the verbiage alone. It is for all practical purposes, just a cog in the Amsoil hype machine.

Thought I'd just mention those two little observations of mine. JMHO, I'm not a lawyer.