I live in Arizona and use 15W-50 Mobil 1. But guess what? I also used it in Minnesota!
It should be noted that the so-called economy oils (5W anything) recommended by big auto companies will hardly show any improvement in fuel mileage over higher viscosity oils and the real reason Ford and others recommend this crap is to permit them to report higher CAFE averages to the US government. Ford has now gone so far as to recommend 5W20!
German car manufacturers have stuck to recommending oil that's best for the customer.
I believe that M-B now recommends using 0W-40 synthetics in any gasoline engine manufactured since 1998 and 10W-40 and heavier synthetics in earlier cars. I believe they specifically recommend not using 0W30, 5W-30 or 10W-30 even if it's a synthetic.