Not sure what you're saying there, but as far as covering southern NA, it's a one size fits all statement.

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).

So, we're thinking the explanation is, what?

Fuel economy / Emissions standards is my guess, but who knows?

Hmm, maybe on cold starts i guess?

I'm pretty certain it's 10w30 in England, like most stuff there, it was 10w40, but seems to h ave moved to the 30 in recent years. However, i know that typically the winters don't get as cold there, even compared to oregon.

Not on track with the last few comments, but it is about oil :-)
Just Changed my oil at just over 5k miles. Have only used Nissan Ester on the last change at 1850 miles. I went with a K&N filter, a filter mag, magnetic drain plug and Penzoil Ultra. I only have about 30 miles on the new oil, but I am very pleased. Engine is quiter, smoother, and feels a bit stronger. The stronger may just be because its smoother, so it catches me off gaurd. Anyhow, overall, I am very pleased with the Penzoil Ultra (no esters).