Have owned relatives of the old Vauxhall, and it did say to change the back axle oil ‘only at overhaul’.Vauxhall deleted the requirement to change back axle oil (and the drain plug?)
There was no set ‘overhaul’ to do, it was just that in case the differential got taken out for repair, it was the ‘opportunity’ to change the oil.
The oil was usually a good black, but it didn’t matter because it’s always cool in there.
The book said to remove the lowest differential mounting bolt to drain the oil.
That oil, SAE 90, is the most tenacious viscous matter you could find.
After clean-up, you could still feel it in your hands for days after, and it smelled like cat’s doo.