Hey awdftw, I think people should be running the 10W30 as Ken quotes from Subaru. You are right that Subaru doesn't recommend any of the 40 weights.
Personally I use 10W30 in my WRX all year round but if I lived in a colder climate I'd use 5W30 in the winter....but I'd definitely never play around with any 0 weight oils and never a 40 viscosity of anything unless running in extremely hot temps.