Maybe @morbo knows the life of like 80w gear oil from the 60’s but I don’t see any reason to service it.
While I used to work with petrochemical engineers, I didn't find their particular engineering field was my speed. I did enjoy engineered resins and polymer engineering, but I was already doing software development for my career after leaving EE and the power industry. My knowledge of petrochemical products and their properties is weak. But, my educated guess is that 80w lubricants in a closed system would have a long lifetime if oxygen and moisture do not infiltrate the system. A heavily greased rear end that's never been cracked open probably has a long service life due to the system being mechanically simple and forgiving. A "pumpkin" from that time would be quite simplistic in that it was just a few gears rather than a modern limited-slip differential with more moving parts in it. Simplicity seems to be the key here for longevity. It's not a high precision component and that works in its favor for sure.
Pumpkins of the time were not Swiss watches. That’s for sure!