Seems that some 2t oil is a bit "difficult" even when fresh. Some TCW3 outboard oils can't be mixed with other tcw3 oil without gelling for example... I have used some husqvarna oil for the weeder and chainsaw, the oil was ca 15 yrs old but unopened- No problems at all. Anther bottle of same oil, opened, showed some strange lumps floating around. Probably moisture ruining it.
I suppose that many oils would be ok even if old if stored properly. The only thing I can think of is the additives deteriorating or dropping to the bottom of the bottle. The oil itself is not breaking down over time, ask the dinos...
Look at the oil. If it looks like new, use it. Else... dump it.