Yes, very interesting exchange of opinions and it does contribute to further educating us. It doesn't solve the thin vs thick debate though, which I am currently on the fence.
On one hand I want the thinnest oil for my application. On the other hand, I want it thick enough to give adequate protection for my engine. I guess other than the cooling system normalizing the temperature to which oil viscosity is affected, shouldn't we look at ambient temperature as well?
If Gokhan wants to use 15W-40 for his car, I would say it's doable since he lives in a warm climate. But, this would not be advisable for a colder climate like Canada or winter in NA.
Ambient temperature under the hood of the car at -32f is very different from 70f, and that has to affect engine temperature that's not reached by the coolant, which then affects oil viscosity. So, thicker would be better in warmer climates than colder ones given the specified ranges in the owners manual. My point is, shouldn't we just follow those ranges given that the engine was built for such tolerance? Also, perhaps the thick vs thin debate overshadows the tight vs loose characteristics of an engine to which we should be paying more attention to in deciding oil viscosity among other things.