The manufacturers say 4 years, most of the additives start to fall out of suspension and cannot be re-added. People have used old oil and have not had problems but sometimes things do go wrong. In aviation, 4 years from both Shell and Phillips, expiration date is on the box. Had a seminar on oils and the shell engineer explained what happens when oils and the additives separated. It may not harm a beater but I would not use old oil in anything of value or something that is hard to replace. I don't have a dog in this fight but that is what the literature says. Tim