A point of clarification, the "W" part of the viscosity grade designation indicates cold temperature performance only. The second number is the operating temperature (210*F) viscosity. A 5W-50 is the exact same oil as a 20W-50 at operating temperature, however the 5W-X will flow a lot better cold - not a bad thing, but unnecessary for mild temperatures like So-Cal or summer anywhere else.
The #1 benefit of synthetics is they do not oxidize as readily as conventional oil, and that keeps your motor clean of deposits. The #2 benefit is they naturally have a broader viscosity index. That means they can be formulated with less viscosity improving additives, that eventually shear down over time.
