Doesn't need to be. So long as the oil meets all manufactures specs they are good to go. Brands like the ones you mentioned usually go over and above the specs so there is no issue using any of them.
IT gets cold here in TX too; not overly cold, but near freezing for many months. And over 100 in summer for months as well. As you probably know, the great majority of engine wear is during cold starts. And as you should know, even starting your car in 100-deg weather, oil is still too thick to lubricate properly (it should be near 200-deg for that). A thinner synthetic oil when cold has ZERO drawbacks, and probably for the reason stated above, most manufacturers are moving to 0/xx oils now. And most manufacturers now approve going from 5 to 0 on the first number, including Ford.
: The Borg Warner cam phaser issues (which we have on the intake cams) are reportedly due to lack of lubrication when engine is cold. Since 0/xx starts lubricating quicker than 5/xx, using it will hopefully eliminate any future issues. Hope that answers your question .