Which viscosity should I buy: 0W-20, 5W-30, or 10W-30?

Use the exact viscosity printed in your owner's manual or oil cap. 0W-20 is common in newer fuel-economy-focused engines and cold climates, 5W-30 is the most common all-around grade, and 10W-30 appears in many older or higher-mileage vehicles. The wrong viscosity can affect fuel economy and warranty coverage.