this is also my experience on mobil1 15w50 synthetic oil. i have much improved fuel consumption something like 11+km liter after 10,000km. when i switched to using 10w40 oil, i have better consumption right from the beginning. this suggests the engine should use w40 oil for better efficiency.
if you take care of your engines, 10w40 is about the right viscosity on BMW engines. another cheap and good oil to use is RRR 10W40 fully synthetic. very good and cheap from UK. only $30 for 5 liters at Carrefour.
for those thinking w60 oil is better for protection, you will have to undo your traditional thinking. modern engines use better oil. new ones even use w30 or w20 oils. so w40 is about the right one, even if you redline frequently.