I have owned 2 BMW's and when runnign the 6cly motors, I can strongly suggest a 5w40 oil.
My current car was run on 15w50 by the PO for a good 2-3 years, with 10w30 in the winter.
When I got the car, I noticed that it ran MUCH BETTER on Delvac1 5w40. The car ran smoother on start-up, and accelerated quicker when revving.
On my old m20 motor where it's has solid lifters and requires valve adjustments ever so often, I noticed that how the oil is performing will have a direct impact on the lifter sound the engine is making. With the 15w50, the idle was rough and there was excessive noise when warming up. It was kinda like I could feel the crank being slowed as it went thru the oil.
After I switched to the 5w40, the engine was much quieter at startup and ran substancially smoother during warm-up. Also the motor held a very attractive (to me) softened valve train noise (some noise is always expected with a solid lifter setup).
When consideirng a different oil for my e30, I thoth anyting rate 5w wold be too thin. But consideirng I'm not burning or dripping any oil, made this oil a great candidate!!!!! i strongly recommend this oil for anyone running anything larger than a 2.3 bmw 6 cyl!!!! tho in some areas it's a bit harder to locate it. If you call the Mobil 1800 # and they'll gladly locate your nearest dealer!!