Mercedes lets you use 30 weight or 40 weight oil. Within each, there is already a range of thickness at operating temp. The point is, is the oil strong enough to do it's job if it passes certain tests. And the answer is that as technology has improved, yes thinner oil has gotten more capable.
The engineers are in one way no different to those who are against thinner oils. They start with the assumption that the thicker oil has more strength. But from that point on, they differ from the amateur skeptics because they go and run tests to see whether a thinner oil can hold up over hundreds of thousands of miles. So, companies like Ford, Toyota and Honda have all established their engines can take thinner oil.
It's entirely believable that thinner oils are fine because people are always innovating. None of you, even the oldies, are getting their comments on this forum through punch tape and computers with valves are you? There has been something called progress taking place ..........