I can confirm MD's thought that this stuff can make your head hurt, but here are some numbers for you.

The SAE J300 viscosity standard for the 0W is a result of testing the maximum viscosity at -30C
The SAE J300 viscosity standard for the 5W is a result of testing the maximum viscosity at -25C

That isn't to say that the maximum viscosity is the ideal viscosity, just that as you exceed it it is harder for the starter motor to turn the engine over. You will very quickly get a few degrees of heat into the oil, and so raise it above these temperatures. What matters is the wear during that (short) time, and the ability of the starter/battery to crank the engine fast enough for it to catch.

I would also think about the range of viscosity in the oil. A 0-40 generally has more viscosity modifiers than a 5-30, and/or a different basestock, just to get the range of viscosity standards. Too many viscosity modifiers means that the oil changes in other characteristics as well, not always to the better.

All that said, I would use a 0W-XX oil at -30C, to get the cranking speed.

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