Well, what I mean is: if it was really about weather, they could have subdivided things differently. North America = colder doesn't seem like a logical way to break things up given that only a small portion of North America's population is in a colder climate that parts of Europe. If this were all about temps, they could have lumped everyone in the non-NA category and told them to use the handy chart listed there (which says the 5W-X's go to somewhere below -22F, and the 10W-X's are good down to -4F).
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