I think most oils have a shelf life of about 7 yrs. before you need to be cautious. There's still alot of folks using MO2&MO3 German Castrol around. Just curious, how did you forget you had it? Do you keep a big stash of oil lying around?
I'd pop the top,pour one in something clear so I could get a good look at it and if it looked fine I would run it in something if it was nothing more than my lawnmower. I don't see why you couldn't.