I've heard all kinds of numbers for this. Two years seems awfully short. Five years looks like a good conservative estimate, but ten years probably is workable when properly stored (avoid freezing temperatures and high heat, so no garage storage). I'd try to stay within roughly 5 years. If you buy a new car and have 5 year old oil, it will not meet spec (e.g., SJ or SL oil and manufacturer specifys SM).