I read a Tech article that basically said the following.

Temperature controls the viscosity of the modern multi-viscosity oils.  Oil claiming to be 10w 40 is 10 weight oil (at 32f degrees) with  non-lubricating molecules that expand to make the equal to 40 weight oil at a given operating temperature. The coolant systems in most modern cars are designed to operate at over 200 degrees forbetter fuel milage. To match the modern operating coolant temperatures the oil reaches the high end of the read out when most cars are running 200 degrees. The actual temperature of the oil when it thickens to 40 weight will be more that 200 degrees.

There are a lot of shade tree engineers advocating running 165 degree thermostats or none at all. This canl make the motor oil run at a cooler temperature and effect the viscosity of the oil.

The nomenclature describing synthetic oil has been revised. Some dino oil is now being called synthetic. A new refining process has modified some ‘dino’ oil to the point that it nearly as heat resistant as the real full synthetic. At any rate motor oil is only as good as the additives in it and they do wear out.