here's MY thinking...
Once broken in, here's what i plan to do and why.
I wil run synthetic because it protects better. Period. If you are content to go cheap, then fine, can't fault you there-- money ain't free!
Synthetic is better in the winter because it flows much sooner. Better in the summer because it can handle heat a lot better. Better for your turbo, which runs over 200K rpm at times.
15W40 synthetic year round is what I will use. the 15W over 5W is important to me. Look at the pour points of most synthetics, and you will see that even a 15W pours as well cold as a 5W or 0W. If it still pumps, I feel a thicker oil is better-- more likely to stay in the rings and high shear areas of the engine. it's also more likely to stay in the engine instead of all draining into the pan, meaning that your starts are much easier on the engine.
the possible downside to running a synthetic that's too thick is reduced economy-- that's it. That pales compared to the possible ring, bearing, and other damage that can happen when the oil is too thin.
I am glad you guys running 5W-30 Amsoil are happy, but I will always run a 15W-40, and it will be synthetic once I get broken in...
HOHN