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Does motor oil go bad just sitting?


Me and me Dad have this discussion quite a bit. Can motor oil sitting in an engine when it's not needing a change harm the engine? Most oil changes recommend xxxxxx miles or every 6 month etc. But why do you need to change it say in the 6 months if you only have 1,000 miles on it? Does it natually break down just sitting? I mean that is what makes oil go bad is from the heat and the viscosity. The molecules will tend to break down with heat, so that scientifically makes sense. But how about it just sitting there if it's still somewhat fresh? I mean does oil just sitting on a shelf expire? I don't seem to remember seeing an expiration date on it. Then there is this saying that you hear, "That oil was in the ground for 100 million years, what’s six more months?”. So what is your take on the matter? Is it all a marketing ploy? Is there truth behind it? Can oil just sitting in an engine with no activity expire and go bad? I did some searching today and found this article, somewhat similar to what I am asking.

Can 10 year old oil go bad?