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Yep, the "W" cold grade is a bit more flexible than the non "W" number. From what I understand 0W-30, 5W-30, 10W-30 and even straight 30 all have the same viscosity requirements at 212* F (100* C) and 305* F (150* C). But when it's colder is when the "W" grade becomes important.
I have a Chevy Astro owner's manual here with me. It states that for this particular engine 10W-30 is ok for summer use, but not winter use. 5W-30 is fine for all year. But if temperatures are continuously below 0* F then it recommends 0W-30.
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