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Regardless of what type of oil you use (conventional, synthetic or synthetic blend), all motor oils eventually wear out and have to be changed (actually, it’s the additives that wear out more so than the oil). As the miles add up, motor oil loses viscosity and gets dirty. The oil no longer has the same viscosity range it had when it was new, and it contains a lot of gunk (moisture and acids from combustion blow by, soot, dirt and particles of metal from normal wear). You can’t really tell much about the condition of the oil by its appearance alone because most oil turns dark brown or black after a few hundred miles of use.
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The oil filter will trap most of the solid contaminants, and the Positive Crankcase Ventilation (PCV) system will siphon off most of the moisture and blow by vapors, if the engine gets hot enough and runs long enough to boil the contaminants out of the oil. Even so, after several thousand miles of driving many of the essential additives in the oil that control viscosity, oxidation, wear and corrosion are badly depleted. At this point, the oil begins to break down and provides much less lubrication and protection than when it was new.
If the oil is not changed, the oil may start to gel or form engine-damaging varnish and sludge deposits. Eventually this can cause your engine to fail! Oil sludge can damage engine bearings, piston rings, cylinder walls, valve guides and lifters.

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