uuhhhh i might get a bit technical here....the reason we change our oil is because motor oil has a "characteristic" called Viscosity, which refers to an oils thickness. As the oil wears over time it loses it's Viscosity (thickness) and becomes thinner, this is why some of us experience tappet/ticking noises when we start our cars up. The thinner the oil gets, the more friction there is on moving components, and whenever you create more friction, you also create more wear, brake pads are a good example as they create friction between the pads and the disk rotor to slow or stop the vehicle, hence why we replace brake pads all the time. So when more friction is created, more wear is also created, which results in major engine damage/wear. 0W50 is a fairly thin oil, and personally i wouldnt put it in my car. Thinner oil like 5W30 is used on brand new cars such as Ford Mondeo's and Ford Focus's, i use 10W30 on my car because it's not too thin and it's not too thick either, i think something like 20W40 would be too thick, you want to give your engine compnents more friction, but i THINK giving it more friction could lead to poor engine performance. This is why i use 10W30, not to thick, not too thin and it's what's recommended in my service guide