There is no difference in cold cranking between a 10W-40 and a 10W-30. If you want a thinner oil for winter you need to go with a lower ?W, 5W or 0W.
The number before the W is the cold weather viscosity of the oil at 0°F, the other number is the viscosity at 210°F.
Oil weight, or viscosity, refers to how thick or thin the oil is. The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).
Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating (example: 10W), and oils that meet the high ratings have no letter (example SAE 30). An Oil is rated for viscosity by heating it to a specified temperature, and then allowing it to flow out of a specifically sized hole. Its viscosity rating is determined by the length of time it takes to flow out of the hole. If it flows quickly, it gets a low rating. If it flows slowly, it gets a high rating.
Engines need oil that is thin enough for cold starts and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils e.g. 0W-40, 10W-30. These oils meet SAE specifications for the low temperature requirements of a light oil and the high temperature requirements of a heavy oil.