5W30 vs 10W40: Which Engine Oil Boosts Performance & Fuel Economy
5W30 and 10W40 are multi-grade motor oils defined by their SAE viscosity numbers: the “W” winter-flow rating and the second number for high-temp thickness. 5W30 stays thinner in cold starts; 10W40 remains thicker when hot, forming a heavier film on metal parts.
Drivers grab whichever jug is on sale and wonder why their mpg or engine noise changed. Mechanics see this daily: the same car may arrive with 10W40 after a quick-lube upsell, then complain of sluggish morning starts or a ticking valve train.
Key Differences
5W30 pours at –35 °C and thins to 9.3–12.5 cSt at 100 °C. 10W40 starts at –30 °C but stays 12.5–16.3 cSt when hot. This means 5W30 reduces cold-start friction, boosting fuel economy 1–2 % in modern engines, while 10W40’s thicker film suits hotter climates and older, worn bearings.
Which One Should You Choose?
Follow the cap stamp first. If it says 5W30, stick with it for warranty and mpg. If your ride has 100k+ miles, minor seepage, or you tow in 100 °F heat, 10W40 can quiet lifters and cut consumption. Never mix weights expecting a “best-of”; choose one and change on schedule.
Can I swap mid-season?
Yes, but only at the next oil change; mixing viscosities dilutes additives and can void warranty.
Does thicker oil protect more?
Only if clearances are large; modern engines with tight tolerances run hotter and rely on 5W30’s faster flow.