I am a strong supporter in using synthetic oil and lubricants.
However, i will not use it in my stock A-833 gear box because it is too slippery and the transmission wasn't designed for it.
I have tried it, and the gears clash when you are trying to shift at any speed.......
It would take a lot of modifications that i would not know how to do properly, so i use the recommended oil in this case.
That being said, i use synthetics exclusively everywhere else on the car.
With the statement you are making, we all should run a modern drive train with EFI etc.
Points ignitions, carburetors, bias ply tires, and dino oil is what a lot of these cars were run with back in the day.
Are they totally lousy? No.
Let's keep it in perspective here........