Hi.
@ quest...please keep in mind that just because a 5W-20 supposedly protects most engines just as well as a 5w30, does not mean it protects every engine.
An oil is specified within the manufacturer's limits. Things like the usable life of the vehicle, fuel economy, wear protection, etc. are all factors when a manufacturer specifies an oil. This is done through extensive testing over the years, on the streets and in the labs. Statistics are added up, and if things fall within range, then an oil specification will be released.
For example, there are some cases where the same engine of several years ago is specified with a lower viscosity oil in the present day.
Basically, the manufacturer has done the testing to conclude that it would be safe to do so.
However, a chemist will always revert back to the molecular structure and bonds between the molecules to examine an oil's capabilities and limits. And there is a simple fact that presents itself each time oils are tested; higher viscosity oils still retain higher film strength at higher temperatures.
Now, the question becomes...will the engine experience those limits? If the answer is 90% no, then it is logical to specify a lower viscosity oil. This way, the manufacturer can achieve minor fuel economy improvements and avoid costly testing to redevelop the engine.
It is good that many engines last 100,000+ miles when they are specified with 5W-20 oils. That shows not only how "good" the lubricant is, but also the engine operating environment.
In motorsports, we rarely use a high viscosity oil unless we are after durability. Usually, we throw in something around 0W-5W oils simply because there is less fluid drag, allowing the engine to maximize power. This is incredibly important if the rules regulate the car's weight, engine design, and aerodynamics. We need every advantage. However, what this also means is less durability. We rebuild everything after the race. To quote Mr. Porsche, the perfect race car crosses the finish line first and then falls to pieces.
However, most consumers don't have this in their objectives. My opinion may be moot, but I still believe that a higher viscosity oil still provides higher film strength at higher temperatures, relative to a lower viscosity oil. It's just fluid mechanics in action.
This also does not means that a higher viscosity oil is a wonder oil. It too has its limits, and that's also why we see engines fail. Now if that's the case, what would happen if we used a lower viscosity oil in the engine?
To recap, a fluid is specified within a limit. However, 95% of the reason why manufacturers have specified lower viscosity oils is because of the fuel economy benefits; they can market this both to the federal government and to the consumer. 95% of the people that go in to buy the car, see the the window sticker rather than the viscosity recommendation on the oil cap.
And if the manufacturer expects the car to last a certain amount of time, within a certain amount of mileage, then it would be logical to specify fluids accordingly.
