You can also use 5W20 if you want, per a Toyota TSB.
If you are using synthetic anyway, 0W30 is theoretically better than 5W30. The viscosity when it's warm is the same, but when it's cold (even summer daytime temperature may be cold for the oil) 0W30 is closer to what the engine wants than 5W30. 0W30 is never thinner than what the engine wants at operating temperature, so it's safe even in the summer. For example, see the bottom of post #2 here:
VQ Oil Analysis and Info - MY350Z.COM Forums
Why don't all cars call for 0W30 or 0W20? They are starting to (like the 5th gen), but 0WXX oils were not common until recently and they are still more expensive than conventional oil, which isn't available in 0WXX versions as far as I know.
I'm going with either 0W30 or 0W20 for mine next time.