Some quick facts. Oil gets thinner and flows easier when it's heated up and is thicker (flows less) when cold (starting). Viscosity is a measure of resistance to liquid flow at a specific temperature and pressure. Multi-grade oils optimize the characteristics of the engine oil by effectively increasing viscosity (resistance to flow and thinning) at higher temps and lower viscosity at lower temps.
I use Mobile Synthetic 0W-30 for SUVs which has a wider temp range vs. 5W-30 even though I live in Southern, CA. I chose this basically to help with slightly better gas mileage. The multi-grade 0W-30 for SUVs maintains the same protection as the OEM 5W-30 at higher temperatures but better at lower temps.
So, when you shake a bottle of 0W-30 vs say, 10W-40 (even 5W-30) you'll notice that when "cold" on the shelf, the oil feels lighter and flows easier. Try it! 0W-30 allows for easier starts and better initial protection until the engine is at running temperature which in turn saves on gas by allowing the engine to run more efficiently for it's first few minutes on start-up everytime/everyday. A good friend (auto enthusiast who rebuilds his own engines) back in high school explained this to me in this manner.
Btw, Porsche TTs, GT1s/2s/3s, RS', and Mercedes AMGs use Mobile One Synthetic 0W-40 OEM ( a wider temp.) because of the potential capability and extreme 0-200+MPH
sustained conditions that these vehicles may often see on the autobahn and extreme climates as uber performance vehicles. Not to mention the much greater heat the engines generate themselves. Mobile One Syn is also listed as the highest "SJ" API service rated (range SA-SJ) meaning that they have the highest integrity of not changing in chemical structure (oxidizing, sludging over time or foaming), retaining deposits, etc.
General information on multi-grade oil viscosities (SAE viscosity grades):
http://www.mattsauto.com/oiltips.html
http://auto.indiamart.com/auto-consumab ... osity.html
Comprehensive API Service Ratings:
http://www.performanceoiltechnology.com ... ations.htm