You will not find any clear answers but you may want to check out this website:
http://www.bobistheoilguy.com/forums...stlist&Board=1
Everything I have read suggests that the frequency of oil changes is a LOT more important than what oil you use. I did an initial change at 1600 miles (I was on a trip when 1200 rolled around) and another at about 5,000 miles. I will only be around 7K miles when my "free" oil change comes due at one year. I used BMW oil both times and plan to continue to use it. I only drive my 128i vert on nice days since I have a perfectly good SUV also sitting in the garage. I need to just go to a six month and 12 month oil change which will put me at about 3500 mile intervals. That is low but more frequent oil changes just wastes a little money on oil and filters - and time is also a variable since oil breaks down due to both usage and time.
There was a link somewhere I read, I do not seem to have bookmarked, that was pretty good arguing that the upper viscosity - the viscosity when the engine is warmed up - should be selected to give you the operating oil pressure you want. Pressure is an indicator of flow. Too much oil flow (too little oil pressure) is less of an issue than too little oil flow (too high an oil pressure) because removing heat and keeping fresh oil in the bearings is a key ingredient to longevity. The main argument of the article is that you need minimized viscosity for startup. The author argued that most wear occurs when oil temperature is low, so viscosity is high, so flow is low. That interval is minimized by using 0 or 5W as the cold viscosity. The 30 versus 40 for the operating viscosity is probably not very important. I would look at 40W more for an old engine where the bearings are a bit worn and the passages for oil flow are thus bigger. So a little thicker oil may be OK.
Jim