That's where most people do not understand the workings of the engine and then wonder why they can't stretch the oil change intervals. It has nothing to do with the degrading of the engine oil but rather the contamination of the engine oil that shortens the oil change intervals. BMW tried their EURO standard 25k km oil change intervals here and ended with slugging issues and dead engines.

The oil itself will last a very long time if there are no contaminants. For example, gear oil works harder than normal engine oil but last much longer as there are no contaminants being introduced to the system. With every combustion of the engine, blow-bys leak past the piston rings and into the engine. Every stroke of the piston, you are introducing petrol, moisture, acid and carbon into the tiny 4-5L capacity of engine oil. All these contaminants has nowhere to go and resides in the engine oil. The oil filter is only good to remove large particles of contaminants but not things like water and acid.

I change my full syn every 5000km not because the oil is dead but the oil is dirty. Here are some numbers to crunch.. if you drive 5000km at 100km/h, at 3000rpm, it will take you 50 hours to complete. 50 x 60 x 3000 x 2 (each rpm is 2 strokes, up and down) = 18,000,000 strokes. How much junk do you think would have gone past the piston rings after 18 million strokes up and down? That is only 5000km at a constant speed.

Added:  Oh yeah.. I forgot.. that is only for 1 cylinder.. if you have 4 cylinders, multiple that by 4.