"Synthetic" oils (PAO or group 3 petroleum) need less

viscosity additives than the traditional petroleum oils to keep the viscosity at high temperatures. These viscosity additivies will break down gradually over time at the high stress/high temerature environment. So depending how hard the engine was running, the oil viscosity index will drop when the mileage is adding up. For example, at 3000 miles, the 5W-30 oil may become 5W-20 in some engines. "Synthetic" oils will hold up better becuase less additive is needed to keep the viscosity. Synthetic PAO oils is even better because the base oil itself is near 5W-30 index already, for example. There is no additive needed for the PAOs in this case. And the PAO molecular are all identical in the oil, the oxidation of the oil will not change the oil property very much.That is why synthetical oil can last longer. And the PAO synthetic oils will last even longer.