What is the primary purpose of using multi-grade engine oil (e.g., SAE 15W-40) in automotive engines?

1. To increase fuel consumption by maintaining a high viscosity index.
2. To maintain a relatively stable viscosity across a wide range of temperatures (thin enough for cold starts, thick enough for hot operations).
3. To lower the boiling point of the engine oil to assist in heat dissipation.
4. To ensure the oil remains thin at high temperatures and thick at low temperatures.