10W30 vs 5W40: Which Oil Grade Boosts Engine Performance & Fuel Economy?
10W30 and 5W40 are multi-grade engine oils: the first number (10 or 5) measures cold-flow viscosity, the second (30 or 40) shows thickness at 100 °C. Drivers mix them up because the numbers feel close—yet one swap can trigger rough starts in winter or extra drag on hot highways, costing fuel and performance. Key Differences…