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
5W40 pours faster at –30 °C, giving instant lubrication in frosty mornings; 10W30 thickens sooner, ideal above –20 °C. At high temps, 5W40’s 40-weight film shields turbo engines; 10W30’s lighter 30 weight cuts friction for NA motors, nudging mpg up 1–2 %.
Which One Should You Choose?
Pick 5W40 for high-mileage, turbo, or towing rigs in mixed climates. Choose 10W30 for newer, naturally-aspirated cars in moderate zones and strict fuel-economy targets. Always match the manual first.
Can I top up 10W30 with 5W40?
Short-term, yes, but viscosity mismatch may lower mpg and void warranty. Flush and refill with the spec grade ASAP.
Will 5W40 improve winter starts?
Yes, its lower “5W” rating flows faster below freezing, cutting start-up wear versus 10W30.
Does 10W30 save fuel?
In ideal temps, the thinner hot grade reduces drag, adding 1–2 % mpg versus heavier 40-weight oils.