The important point about synthetic is the thermal protection it affords when the turbo gets hot. The first number refers to it's cold viscosity. Either way, you can't go wrong. Some even use 0W30. I use the 5W30 in winter for the better cold-weather flow characteristics (pour point, viscosity, etc.). Here in the northeast it's been a cold winter, but yet nowhere near cold enough to affect flow characteristics of the heavier 10W30. Still, I like the notion that the thinner lubricant reaches journals an instant faster on start up.

(Victims of convenience are we. Race cars and many antique cars require the oil pump to be started several seconds before ignition, just to ensure that there are no dry surfaces on start up. Yet when we turn our key, it's click-vroom! Makes me wonder.)
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David
98 S70 T5SE // Black, misc mods (mostly lighting), red calipers
92 940GLE // Hella Micro DE foglights