We usually recommend to adhere to manufacturer’s recommendations on oil viscosity. With that said, I personally prefer to go to the heavy side of the recommended oil viscosity for a given climate range. At the shop here in Dayton, Ohio we use Motul 5W-40 for all late model German cars (except BMW M series) for all four seasons. 5W-40 easily covers the viscosity needs for these cars in nearly any conceivable temperature we are likely to see in our part of the world. For BMW M Series we use Lubro Moly 10W-60 year round because that is what BMW recommends. The only oil related issue that I disagree with the manufacturers on is the extended change interval now used by all the German makes. Porsche recommends 20,000 miles or once annually between changes, BMW and Mercedes are both using condition based intervals that result in between 15 and 18 thousand mile changes. We recommend changing the oil in half the time the manufacturer recommends or at about 7500 miles.