I have 2000 E430 I bought new in April 2000. The car had M1 0W40 the first 8 years/90k miles using FSS for OCI's at around 12k miles/1 year.
Last spring (2008) I tried PP 5W20 and found that the engine noise was much quieter with thinner oil. I went to Vegas last July 04 and got stuck at Death Valley for 3 hours with ambient temp above 120F, but the coolant temperature was never above 98C with air conditioning ran at full blast. The PP color was very dark after only 1k mile and almost black after 3k miles, I think PP 5W20 do some cleaning after 90 k miles with long OCI's even with good synthetic oil such as M1 0W40.
Because of this severe condition, I changed PP out with only 6k miles with Delo 15W40. Engine noise was as noisy as with M1 0W40 and gas mileage drop down as with M1 0W40. Gas mileage was 4-5% better with PP 5W20.
Last Dec I changed to 5qts M1 0W20 + 3qts M1 10W30, engine noise is reduced substantial compares with either M1 0W40 or Delo 15W40, about the same as with PP 5W20. Gas mileage is back to about PP 5W20, I just made a 140 miles round trip with 4 people and luggage, about 80% highway and got 26.6 mpg with average speed of 46 mph. The EPA rating is 18/24 City/Hwy.
The conclusion is, my particular E430 like thinner oil. Engine is much quieter and accelerated much faster while consumed much less fuel.
My next OCI will be with Synpower 5W20 I bought on sale (and rebates) for less than $1/qt. All the oils I have in stock were bought on sale with rebate(s), such that I have several brand names and I don't think brand matter. The main effect on engine performance is viscosity and I and my engines love thinner oil.