1. The company recommends 5W-20, so I think theory may know more then
even a very smart dealer.

2. In theory 5W-20, 5W-30 and 5W-15 should be the same viscosity at
low temperatures, so start up protection should be the same, where as
any 10W-XX oil would have a significantly higher viscosity. This
could starve the engine for oil if the pump and oil gallies are
designed for a lower viscosity. It will also add to friction during
warm up, reducing gas mileage on very short trips.

3. 5W-20 oil will be less viscous at normal running temperature than
5W-30, 10W-30, or 10W-40. This should reduce friction as long as the
engine is designed to not need the higher viscosity for protection.
In turn this should provide better gas mileage as long as the engine
machining is very precise.

4. Most modern engines are designed to run in a very narrow
temperature range once warmed up. The ambient temperature has very
little to do with the steady-state temperatures within the engine.
This being the case, there is a significant reduction in the
requirement to run a higher viscosity oil in warm weather or warm
climates.

For these 4 reasons I would stick with the MANUFACTURERS
recommendations on oil viscosity.

A perfect oil would keep the same viscosity at any temperature. The
engine is designed to run with oil in the viscosity range of no
thicker than cold 5 weight, and no thinner than hot 20 weight. When
hot your 10W-30 will be well within this range, when cold it will be
a good bit thicker. At least use a 5W-30, if 5W-20 is not available
in the brand oil you like.