In my case,I had spilled most of the oil while transporting the welder [it tipped over,you should have seen my van ! ],so I had to replace it anyway.
But you ask is there a problem with old oil ? Well I worked for many years for a firm who made transformers,from 3KVA to 10MVA and when some of the smaller ones [5 to 15 KVA ]came back for repair,having been hit by lightning,the coils would be burned to pulp and the oil like tar,and the smell of it !!.
So I would say, yes, there may be a problem with old oil,although I doubt if your welder is likely to be hit by lightning.
Perhaps the other mans oil level was just low,or dirty.