The results of oil testing to accepted industry standards convinced me that Amsoil and Mobil 1 EP are the best for most vehicle. On engines which require frequent changes (like the old Rover/Buick engines), these are not really cost effective, but they do reduce lubrication-related wear.
The "0W" tells us that it will flow well at low temperatures, while the "-50" tells us that it will adhere and provide lubrication at high temps. Down here in Texas, the 15W-50 will be fine, as would 10W-40.
I have not gotten a copy, but my indy shop tells me that the 2003-2005 BMW-engine Rovers have a TSB for a knock, which recommends using 20W-50 oil to resolve the noise. It sounds like the Vanos units on these engines may be subject to a problem which increases sound emissions - which may be helped by higher viscosity oil. I found that my noise decreased with heavier oil.
My reference was for the 2003-2005 BMW engine. And yes, 15W-50 is heavy, but the LRNA TSB recommended 20W-50. I have lost 1 mpg since the oil viscosity change (and another after going to 18 wheels and slightly over-sized Goodyear Silent Armor tires.) I may change to 10W-40 for the remainder of the winter on the next change.