The so called *economy* viscosities are a farce. They are recommended by car manufacturer's so they can report a few tenths better mileage to the government to help their Corporate Average Fuel Economy numbers. You won't notice any difference in economy but you will be compromising long term reliability.
If you own a Mercedes, follow the recommendations in your owner's manual. Mercedes are still more interested in reliability than in avoiding gas guzzler taxes (can't say the same for US and Japanese manufacturer's).
I wouldn't use anything less than 10W-40 in northern California (summer or winter).