Since this post has predictably turned into a discussion of which oil is best, I'll put in my 2 cents worth.
I was first convinced that, based on several on line sources, I should use a heavier weight oil, I used both 15-50 and 20-50 for "flush" oil changes of about 1k miles each (I just got my car and the previous owner went long on the oil change intervals), both to try to get rid of the start up rattle and since I live in Miami Florida where its always hot, to protect against heat breakdown of oil. The heavier weight oils made no demonstrable difference in start up clatter.
Then I read the article below and I am a now a convert to M1 0-40. The long and the short of the article is that oil flow is what is important. You want as much volume going through the motor as possible at all times. Since 0-X weight oil is thinner when cold than a 5, 10, 20-X oil, it will provide more volume while the motor is warming up.
And remember, oil temp lags behind coolant temp during warmup. My Durametric shows that oil temp is about 10 C degrees behing coolant temp until the coolant gets close to 90 C, which takes a good 20 minutes in my car with combined city/highway driving. Once its up to temp, the oil temp is a little less than 10 C hotter than the collant temp, in the 100-105 C. range.
Once the oil is hot, the viscosity in now at 40, which at the oil temps I am running, is more than enough to protect the motor. Now, if I were running my car on the track, oil temps might be significantly higher, so I might consider a heavier oil.
But for the type of driving I do, I am convinced a 0-40 oil will protect the engine better during the crucial warm up, which if you do a lot of short trip city driving, will be a big portion of the time you are driving your car.
http://www.planet-9.com/cayman-boxster-tech/60791-calling-all-engineers-motor-heads.html