Which Motor Oil Is Thicker: 5W-30 or 10W-30?

10W-30 is thicker than 5W-30 at cold temperatures, while both are designed to have similar viscosity once the engine is at operating temperature. Put simply: choose 5W-30 for better cold-start flow; expect little difference between the two when hot, assuming both meet the same industry specifications.

How Viscosity Grades Actually Work

Motor-oil viscosity grades are set by the SAE J300 standard. The number before the “W” (for “winter”) indicates how the oil behaves in cold conditions, affecting cranking and pumping during startup. The number after the “W” describes viscosity at operating temperature (measured at 100°C, with additional high-temperature, high-shear checks around 150°C). Lower “W” numbers flow more easily in the cold; the same “30” hot rating means both oils target a similar viscosity range once warmed up.

Cold-Start Behavior: Where the Difference Matters Most

At lower temperatures, 5W-30 will flow easier and reach critical engine components faster than 10W-30. That’s because the 5W rating is validated at colder test temperatures than the 10W rating, indicating better low-temperature performance.

Below is a quick reference to the cold-temperature test limits defined by SAE J300 that illustrate this difference.

• 5W: Maximum cranking (CCS) viscosity of 6,600 mPa·s at −30°C; maximum pumping (MRV) viscosity of 60,000 mPa·s at −35°C.
• 10W: Maximum cranking (CCS) viscosity of 7,000 mPa·s at −25°C; maximum pumping (MRV) viscosity of 60,000 mPa·s at −30°C.

Because 5W oils meet their limits at lower temperatures, they typically provide easier starts and faster oil circulation in cold climates compared with 10W oils.

Hot Operating Temperature: Largely a Draw

Both 5W-30 and 10W-30 are “30-weight” when hot. Under SAE J300, a 30-grade oil has a kinematic viscosity between 9.3 and 12.5 mm²/s (cSt) at 100°C, and must meet minimum high-temperature high-shear (HTHS) requirements. In practice, individual formulations can vary slightly within that range; some 10W-30s may have marginally higher HTHS than some 5W-30s, but modern synthetics often narrow or eliminate that gap. For most drivers, hot-operating viscosity differences between quality 5W-30 and 10W-30 oils are negligible.

Choosing Between 5W-30 and 10W-30

Consider climate, manufacturer guidance, and your driving profile when deciding which grade to use.

• Cold climates: 5W-30 is preferred for easier starts and faster lubrication during winter or frequent short trips.
• Moderate to warm climates: Either can work if approved by your vehicle maker; 10W-30 is often acceptable where freezing temps are rare.
• Manufacturer specs: Always follow the owner’s manual and required API/ILSAC/ACEA approvals to maintain performance and warranty compliance.
• Older or high-mileage engines: Some drivers see slightly lower oil consumption with 10W-30, but this is engine- and oil-specific.
• Fuel economy: 5W-30 may offer a minor efficiency edge in cold starts; differences at operating temperature are typically small.

If your manual lists both grades for different temperature ranges, use 5W-30 when you expect cold starts and 10W-30 when operating exclusively in warmer conditions—provided both meet the required certifications.

Key Takeaways for Drivers

Here’s what matters most for everyday maintenance decisions.

• Thicker when cold: 10W-30.
• Similar when hot: Both are 30-weight and aim for the same viscosity range at operating temperature.
• Best general-purpose choice in mixed climates: 5W-30, especially for winter starts.
• Always defer to the owner’s manual: Specifications and approvals matter as much as the grade numbers.

Sticking to the correct grade and certification is more important than minor differences between two quality oils of the same hot rating.

Summary

10W-30 is thicker than 5W-30 at cold temperatures, but both target the same viscosity once the engine is hot. For most drivers—especially in colder weather—5W-30 offers better cold-start protection. In warm climates, either can be suitable if your vehicle maker approves both. Always follow your owner’s manual and required oil specifications.