I too had that question a few years back with regards to the synthetic oil I'd had for about 5 years. I don't have any experience with petroleum based oils, although the oil certainly won't go bad the additive package may tend to separate and settle out of the mix. I was told by the Redline rep. that their synthetic oil had no shelf life, and that I should just shake the bottle before using. I've used all that oil up now with no resulting problems. I don't think that I would keep an oil too long though as API ratings change and an oil may become obsolete for the present vehicle application. I may buy a few cases if I'm ordering a brand in that I can't get locally. Other than that I'd buy only what's needed for the change if your brand is always available.