Check this link: Motor Oil 101 - Bob is the Oil Guy and read the whole article, 101 through 201. It's long but well worth it. It will bring you to a whole new understanding of engine oil and how it really works. You will find the answer your looking for and learn that most people are spending way more money on oil than they need to. Like oil changes at the age old standard of 3k miles or 3 months, it's a wast of money! I know crazy, read the article!
Something I learned from it that may apply to your question... Use the lowest first number always, 0w or 5w because it only applies to the start up oil thickness. Whether you are in a cold or hot environment you want the thinest oil at start up you can get to put the least amount of load on the oil system. And stick with the second number that your owners manual recomends, 20, 30, 40 ect. That second number, as I'm sure you know, is the weight or thickness of the oil at normal operating temp.
... Read the article it will speak for it's self!