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

KSP said
First of all, I am not just assuming the failure of this 4-speed manual transmission was from the AMSOIL lubrication. I am saying this lubrication 100% caused the failure… This transmission was professional rebuilt to factory spec’s (With All New Parts) and had less then 1000 miles on the rebuild when a AMSOIL distributor at a local car show convinced me there was advantages of using AMSOIL and sold me this product. After the damage was done AMSOIL was not willing to stand behind their product! As I wrote before, rather then AMSOIL doing the right thing and repairing this transmission, AMSOIL spent more then twice the funds on testing of the oil sample to make themselves out to not be at fault.

I received the following comments from an AMSOIL supplier (I tell all of my customers that if there is a problem, I want to know about it immediately, because I'm there to help resolve any issues.) My reply to this person was if there has never been and issues with the AMSOIL products why would the suppler write this? (No Reply...)

You still say it caused the problem but you don't say why you think that and what the proof was. Brand new rebuilds grenading is nothing new. If the fact that it was a "professional rebuilt to factory spec’s (With All New Parts)" is your main reason for blaming the lube you had no case at all. In fact, as Ace said, having it go so soon after the rebuild makes it even less likely that it was the lube that caused it. When I rebuild something, the first 1000 miles or so is when I am nervous about it flying apart for some reason. If you get that far the parts were good and so was the work. Parts can be bad, even professional mechanics screw up once in a while as well. It is just extremely unlikely (if not impossible) that a bad product (from any manufacturer) would cause a problem for only one customer and not hundreds or even thousands. Lubes are made in huge batches, not a couple quarts at a time. It is FAR more likely that your mechanic screwed up and blamed the lube or maybe you got a bad part. It happened to me. The clutch disc in my almost brand new '87 GLHS flew apart because of metal fatigue. Also, I bought a Mopar Performance 3.87 gearset for the same car that had the bearing journals on the intermediate shaft cut several thousanths to small. If that was not noticed and we had put it together it would not have been long before I needed a broom and mop. And a new transaxle.

Maybe they did not "stand behind their product" or do what you thought was the "right thing" because they knew the lube wasn't the problem and they also knew that if they paid such a claim (that seems insignificant to you) they would open themselves to a possible situation where anyone who heard about the case who broke an engine or transmission would try to get them to pay for it whether they used their products or not and even if they never had to pay a claim the defenses could cost them cubic dollars.