Your owners manual will either recommend one viscosity for all temp ranges, or have a scale recommending the proper viscosity for a range of ambient temps.
The first number on the bottle (5W for example) is the viscosity when the oil is cold. The second number is the viscosity when at operating temp.
Usually, all of the recommended viscosities will have the same second number, as the operating temp of the engine stays constant regardless of ambient temp. The first number may change to a lower one for colder climates, just to ease starting.
Look at the oil recommendation "thermometer" in your vehicle owner's manual. The vehicle/engine manufacturer does have some foggy idea of what's best for your engine and operating conditions.
If 5W-30 synthetic is recommended, where's the beef ? Hell, my F-150 4.6L V-8 uses 5W-20 synthetic oil year-round, has 106,800 miles on it and does not use/lose one drop in 6,500 miles. The STS uses 5W-30 synthetic and gets about the same oil mileage. Not a problem.
5W-30 oil is thinner oil with lighter viscosity (than 10W-40) that creates less drag on the crankshaft, pistons and valve-train. Additionally, the oil pump can pump thinner oil more easily, improving oil circulation. Any increase in fuel economy may not be noticed by the average motorist. Machined internal engine parts are more precise than the parts of 25 years ago. This means that clearances between moving parts are smaller and more exact. Thinner oil such as 5W-30 can flow more freely through the engine while still filling the spaces. Thicker oil (10W-40) is harder to push through the spaces between the parts. This causes the oil pump to work harder, which in turn increases oil pressure while simultaneously decreasing oil volume. A lack of oil volume results in a decrease of lubrication and cooling, which may decrease engine part life.
The lighter viscosity of 5W-30 oil flows faster at start-up compared to higher viscosity oils as 10W-40, which helps reduce engine wear in critical areas by lubricating parts faster. Valve train components at the top of the engine require immediate lubrication at start-up.