Some grades can absorb (adsorb?) moisture from the atmosphere. Especially those with lots of additives.
So after a few years (especially if they’ve been opened) they won’t work as well as new stuff.
Also (i’ve heard) that the additives can react with each other once they start to “go off”, makes the process quicker.
Some engines/gearboxes are very sensitive to this “not so good” oil.
Others don’t care. I’d not want to put ancient oil in a shiny new auto gearbox. But a 10 year old diesel engine. Probably not so bothered. As long as it’s the right grade/spec. And the oils actually been stored somewhere, rather than dumped in the corner of a damp garage.
I had to dispose of about 3000 litres of posh engine oil when one of our logistics guys took a sample from each of half a dozen barrels then left the caps off.
Wouldn’t have been so bad, except the pallets were outside. In december. It rained.
Idiot.