Both numbers are the weight. The first number is the viscosity at 0°F. That's where the W comes from. It designates a winter grade oil. The second number is viscosity at 210°F. It measures how thick oil is at operating temperature. At 210°F, a 10W-40 must fall into a heavier range than a 10W-30, or say a 5W-20. When hot, a 20W-50 will be heavier still. So, for the purpose of helping a problem engine that burns or leaks oil, the second number is the most relevant.
