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). I think I have read that engine oil should be changed every 3k miles or 6 months. How did they come up with those numbers? Probably marketing,

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Yep, brought to you by the same bunch that tell you Pepsi goes bad if it's been on the shelf for too long.

When pigs fly will oil EVER go bad: it's been in the ground for millions of years. We have several cars that get driven maybe once every 3 years with 20 year old oil in them and they run great whenever they get sparked up. If oil really did expire then wouldn't they have a hole eaten out of the oilpan?

Hope that answers your question!

By the way, change oil at 6 or 8 thousand miles, not 3: you're just throwing money away.