I think I know the reasons dave!
I'm pretty sure I know the reasons.
I run Mobil 1 5w-30 Year Round! The reason why I run 5w-30 is because that is what the factory owners manual tells me to!
But seriously though, I ran 10w-30 one time in my car and when the temperature started getting below 25degrees F, when I started the car in the morning, there was a lot of lifter tapping.
After about 2 mornings of that, I changed over to 5w-30. It works great. I've started my car at -15 Degrees F with 5w-30 in the crankcase. The engine is noisy for the first couple of minutes, but there isn't loud lifter tapping.
I would imagine that running a 15w-50 oil would only be worse in cold weather.
Another thing too, I know for a fact that Mobil 1 is Fully Synthetic. What I mean by that is every single molecule in mobil 1 is man-made Olefin/polymer base. This is not true for other fully synthetic oils that still use purified petroleum base stock with synthetic detergents and fiction modifiers and pass them off as being "fully synthetic".
I remember reading by Dr.Fred 4.3 that when it comes to oil grade selection, you want to use the viscosity index that is most appropriate for the temperature the engine will be operating at and then be sure to use the most NARROW range possible for the best in oil efficiency.
It's one of the reasons why 5w-30 will carry a "energy conserving" seal on it's label, but 15w-50 won't.
It's another reason too why some cars these days are requiring the usage of 0w-20 and 5w-20 oils Which is recommended for newer honda and ford engines. Because they are so thin to work into the even tighter tolerances of the engine. I've also seen 0w-40 which is recommended for BMW and Mercedes engines.
Lastly, I'm seeing Fully synthetic "endurance" oil which can go an astonishing 15,000miles between changes. Which IMHO is crazy talk!
With all of the different grades to choose from and since I still drive on the street, I'll stick with what the factory recommends, use synthetic and change it every 3000 miles just to be a fastidious nut-job.