You're missing the point of multi grade oils, Bob- it's only thin at start up, where even 5w doesn't flow immediately through the tight bearing clearances most engines are built with today. Whether the ambient temp is -10 or 105°, at start up the engine is cold and an oil with a lower 1st number will flow, lubricate surfaces and carry away heat sooner than an oil that acts thicker when cold. The current trend with most car manufacturers is to use the same (low first number) weight oil year-round because they've realized that ambient temp doesn't really have much of an affect on an engine's running/performance parameters.
UHM I guess,,UR the expert..Im gonna stick with what the Porsche engineers recommend for my 17 C2S twin turbo 480 HP
im not running this beast on 5W oil