The first number is the cold-flow rating, the second is the hot viscosity. 0W-20 flows faster at startup for better fuel economy in modern engines. 5W-30 provides a thicker hot film for higher-mileage engines and track use. Check your owner’s manual first — the recommended grade is the safe pick. If you drive hard or live somewhere hot, a higher hot rating (40) adds protection at the cost of a tiny fuel penalty.