I used to use the 15w40 A because it was cheaper and I would change my oil every 1500 miles. But after seeing Higher oil temps in the summer, I changed nothing but the oil. Tried Mobile 15w50 ran cooler but hell it was $$$ Then I tried the 5W40 Rotella and My temps dropped back to what the should be. I found out on the Internet, that other BMW guys had experienced the same thing, same with The VW's and Porsche crew.