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There is a vast array of misinformation about oil viscosity and performance, mostly because of competing advertising and marketing claims that try to make a convincing story about "xyz oil is better because (fill in the blank)". Nothing else lights up an automotive forum quite the way an oil thread does. Personally, I believe that's because people's beliefs about oil are like a religion - the actual performance is invisible so it's possible to believe anything you want, based on what someone who purports to be an expert tells you.
A few basic facts:
1. Higher viscosity oil flows slower than thinner oil, so it spends more time in the confined spaces of bearings and so on, so it absorbs more heat and returns to the oil pan hotter than thin oil does. The engine components run cooler with thinner oil.
2. Thicker oil produces a thicker oil film inside bearings and maintains more space between the journals and the bearing shells. That said, so long as they're not touching, it's all good.
3. Oil viscosity lowers with temperature and shearing between close fast-moving metal parts (timing chains are a real factor in this). It's supposed to, and generally the viscosity recovers to original values again when it cools off and comes to rest. To the extent that it doesn't, it's called "permanent shear" and Motorcraft 5w-50 oil has a lot of it. Considering the millions of dollars that both Ford and Conoco Phillips (who make the oil) spend on engineering, its likely that this behavior is exactly what's intended and is not a sign of poor quality (as some commentators contend in order to make their competing products sound "better").
4. Most people think their driving habits are harder on their engine than they actually are. The only thing that's really tough is running the engine hard when it's cold. Other than that, Ford's ECU programming pretty much makes sure that it's well protected when you're driving it as hard as you want or can.
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