My last job was a lubrication chemist. I am no expert but did learn a lot about motor oils. Unless one is driving under severe conditions , such as a desert or north pole, synthetic will be a waste of money. The oil molecule may not break down, but the additives that make the engine last for thousands of miles are depleted after a while, plus the buildup of contaminants. By the way, the W as in 10W-40 means winter viscosity, not weight.