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If you are not testing your oil on a regular basis, then the answer is yes, they do "go bad". They don't suddenly break down but oil health is like bloodwork, it tells you a lot of detailed info about the health of your engine internals.
Oil change intervals should be lengthened only with the use of oil test results that have a known time or mileage interval... otherwise you're just making good guesses with no basis in fact. You should swap out the fluid if you're even under recommended mileage intervals, because you have no way of knowing how much additives were consumed already up to that point.
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