Degradation of the oil happens for two reasons:
- It gets used
- The additive package within the oil breaks down
With it gets used, I'm talking about mileage (or hours for certain applications). Using the motor puts carbon, trace amounts of water, and unspent gas (hydrocarbons) into the oil. This contaminates the oil, which a filter will do a pretty good job of removing the carbon ... until it too gets used up. Water gets into the oil in trace amounts due to blow-by, but evaporates when the engine becomes fully heated. Hydrocarbons will stay in the oil, helping to break down the oil additives.
The main way that some vehicles are able to go further between oil changes is because they have more oil in them. Take for instance some the Mercedes models take 8 to 9 quarts of oil (I believe the Dodge Hemi SRT is the same). With more oil in the crank case, it takes longer for these oils to become degraded due to usage or to a point where they no longer provide protection for the engine as they should.
With additive package, I'm talking about time. After the oil is put into the engine and becomes used (to whatever state), it will start degrading with time. It doesn't matter if there are 5 quarts or 30 quarts, it will all degrade with the same amount of time.
There is a website dedicated to motor oil. For the most part, their information seems pretty good (I have a few misgivings, but this may be opinion on my part). They talk about a couple of things I'll copy over here:
The downside of a mineral based multi-grade oil is that this VII [viscosity index improvers] additive wears out over time and you end up with the original straight 10 grade oil. It will go back to being too thin when hot. It will have a thickness of 6 instead of 10. This may be why Porsche (according to some people) does not want a 0W-30 but rather a 10W-30. If the VII wears out the 0W-30 will ultimately be thinner, a straight 0 grade oil. When the VII is used up in the 10W-30 oil it too is thinner. It goes back to a straight 10 grade oil.
It goes on to say:
Synthetic oils are a whole different story. There is no VI improver added so there is nothing to wear out. The actual oil molecules never wear out. You could almost use the same oil forever. The problem is that there are other additives and they do get used up. I suppose if there was a good way to keep oil clean you could just add a can of additives every 6 months and just change the filter, never changing the oil.
The reality is that motor oils do not need to be changed because they thin with use. It is the eventual thickening that limits the time you may keep oil in your engine. The limit is both time itself (with no motor use) and/or mileage use. The storage of motor oil in your garage, particularly mineral based oils, slowly ages the oil limiting its use later. Do not store huge volumes of oil in your garage that is exposed to extremes of temperature.