It's important to emphasize that wind chill is a "living" phenomenon, experienced only by biologically-living organisms. Inanimate objects, such as your car, will only cool to the temperature read on the thermometer. Blowing wind will only allow these objects to cool more quickly.
That is, whatever temperature you read on your thermometer, that is the temperature your car was starting in, such as -9 degrees Fahrenheit this morning for me.
Any popular 5w oil, dino or syn, should be able to handle such low temperatures.