Even though the common opinion is that engine oil turns black due to slowly absorbing combustion soot in the gasses that blowby the piston rings, that is not the primary reason. The main reason oil turns black is the constant exposure to high heat inside the engine combined with the chemical reactions (mostly oxidation) that are enhanced by mixing with the various gasses that are in the blowby (especially Nitrogen Oxides). Basically the oil is cooking slowly whenever it is running and up-to-temp. If you put engine oil in a frying pan on a stove at temperatures above 250 F, it will turn black after a while just from oxidation.
There are many surfaces inside an engine that are over 250F that the oil flows over like pistons and upper cylinder liners.
As engine oil approaches and goes beyond its normal service life several things happen that make it unsuitable for the engine:
1) The viscosity (oil thickness) changes. Continual exposure to heat tends to thicken the oil and it eventually becomes out-of-spec for your engine. this can increase wear in the engine
2) The detergents that are in the oil get depleted and the things that contaminate the oil continually (like soot) build-up; but the worst problem is that the detergents that are supposed to prevent sludge formation in crankcase stop working. The effect is that the oil starts turning into a grease-like product that will not flow to your bearings
3) The oil becomes corrosive, because the anti-corrosive additives wear out. This can lead to severe wear
4) In cold weather, especially with many short trips, the oil will become diluted with raw fuel and liquid water. This really degrades the lubricant properties of the oil.
As to how long you can go with old oil, generally 7 months or 7500 miles, but the only correct answer is to read your car’s owner’s manual and find the answer in the maintenance section
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