Choosing the right engine oil is one of the highest-impact, lowest-cost maintenance decisions you make for an engine. Viscosity grade (the “5W30” or “10W40” printed on the bottle) tells you how an oil behaves in cold starts and at normal operating temperature.
Below, I’ll explain what those numbers mean according to SAE J300, how the two grades differ in cold and hot conditions, what that means for engine protection, fuel economy, and real-world use — then finish with a clear recommendation framework you can apply to your car.
Contents
What the Numbers Mean (SAE J300 fundamentals)
- SAE J300 is the international classification that defines engine oil viscosity grades. The standard sets the test methods and the required viscosity ranges for each grade.
- In 5W-30:
- “5W” = winter (low-temperature) grade. It specifies the oil’s maximum allowable cold viscosity (measured by cold-cranking simulator and pumping tests) so the oil flows and the engine can be cranked at low ambient temperatures. Lower W numbers = better cold-start flow.
- “30” = the oil’s kinematic viscosity band at 100 °C (approximate engine operating temperature) and an implied HTHS (high-temperature high-shear) floor for load-bearing. For SAE 30 the kinematic viscosity at 100 °C falls in ~9.3–12.5 cSt.
- In 10W-40:
- “10W” = thicker than 5W at low temps (passes cold tests at a warmer temperature than 5W oils).
- “40” = higher kinematic viscosity at 100 °C (≈12.5–16.3 cSt) and usually a higher HTHS floor in practice. That means the oil will be thicker at operating temperature than SAE 30 oils.
Short takeaway: the first number + “W” governs cold-temperature performance; the second number indicates how thick the oil will be at running temperature.
Cold vs Hot Viscosity Explained
- Cold starts
- 5W-30 flows more easily at low ambient temperatures than 10W-40. That reduces crank resistance, improves oil circulation to bearings at start-up, and lowers wear caused by delayed film formation during the first seconds of engine operation. Think of it like syrup vs runny honey: at fridge temperatures, syrup is slow, runny honey flows more — the “W” number tells you which will pour sooner in cold weather.
- At operating temperature
- 10W-40 stays thicker at 100 °C than 5W-30. Thicker oil gives a stronger hydrodynamic film under high loads and high temperatures, but also increases viscous drag. SAE’s HTHS and kinematic viscosity bands define that behavior.
- Prolonged/high-heat use
- Oils are subject to thinning via shear (mechanical breakdown of viscosity improvers) and chemical thinning via oxidation. In severe, sustained high-temperature duty, an oil with higher base viscosity and good shear stability (often 40-grade or specially formulated synthetic 30s) retains protective film thickness better.
Engine Protection and Wear Resistance
Key technical concepts:
- Film strength / hydrodynamic film: At a given load and speed, thicker oil (higher kinematic viscosity/HTHS) produces a thicker fluid film separating metal surfaces. That reduces boundary contact under high loads. 10W-40 generally gives a stronger film at operating temperature than a typical 5W-30.
- Shear stability: Multi-grade oils use viscosity-index improvers (polymers). Under mechanical shear, these polymers can break, lowering the oil’s effective viscosity. A well-formulated synthetic 5W-30 can have excellent shear stability; conversely, a poorly formulated 10W-40 could shear down more than a high-quality 5W-30. Always check HTHS numbers and the product technical data sheet.
- Oxidation resistance: Higher operating temperatures accelerate oxidation; additive packages (antioxidants, detergents) and base oil quality control this. Thicker oils don’t automatically resist oxidation — formulation matters. Good modern synthetics (both 5W-30 and 10W-40) include antioxidant packages and dispersants to manage oxidation and sludge.
- Summary: 10W-40 tends to give a greater margin against metal-to-metal contact in high-heat/high-load situations; 5W-30 reduces start-up wear and often improves fuel economy if the engine was designed around a lower HTHS/kinematic viscosity.
Ideal Applications
- 5W-30 — Best when:
- You live in cold or moderate climates (improves cold-start oiling).
- Your vehicle is a modern passenger car engineered to run on 30-grade oils (many OEMs specify 5W-30 or 0W-20 for fuel economy and emissions systems).
- You want a small fuel economy benefit on highway/city driving (lower viscous losses).
- 10W-40 — Best when:
- You operate in hot climates or do sustained towing, heavy loads, or performance driving, where higher operating temperatures demand a thicker operating film.
- You have a high-mileage or slightly worn engine where greater film to compensate for clearances can reduce oil consumption and noise. (Often chosen for older engines.)
- You run engines that were historically specified with 40-grade oils (some diesel or older petrol engines).
Manufacturer Recommendations
- Always start with the OEM manual. Manufacturers specify viscosity ranges for ambient temperature windows and engine designs — they know the oil pump clearances, bearing tolerances, emissions controls, and warranty needs. SAE grades and API/ACEA specifications are tools to implement an OEM’s guidance.
- When to deviate (carefully):
- If you live outside the OEM’s expected climate extremes (e.g., move a vehicle designed for temperate climates to a very hot region), moving one viscosity step up (e.g., 5W-30 → 10W-40) can be appropriate if the engine condition and oil spec allow it and you accept tradeoffs (fuel economy, possible increased oil pressure). Consult a dealer or service manual and choose a product meeting OEM performance specs (API/ACEA/ILSAC) first.
- Spec compliance: Match both the viscosity grade and performance spec (API SN/CK-4; ACEA A3/B4, C3, etc.) — viscosity alone is not enough. Modern engines often require low SAPS (sulfated ash, phosphorus, sulfur) or specific catalyst-friendly chemistry.
Maintenance Implications
- Oil change intervals
- Intervals depend on oil type (synthetic vs conventional), vehicle duty, and OEM recommendations. A high-quality synthetic 5W-30 or 10W-40 with robust additives often allows the same or longer intervals than conventional oils — but follow OEM interval guidance for warranty reasons.
- Fuel economy
- Thinner oils at operating temperature (lower HTHS/kinematic viscosity) reduce viscous friction, producing modest fuel economy gains. Typical delta: switching from 10W-40 to 5W-30 might yield small improvements (fractional percentage points) — useful over many miles but not dramatic.
- Wear prevention
- For short, cold commutes, 5W-30 reduces start-up wear. For heavy loads/high temps or engines with wear/clearances, 10W-40 can reduce wear during sustained high loads. The oil’s additive package and base-stock quality matter as much as grade.
Side-by-Side Comparison
| Feature / Metric | 5W-30 | 10W-40 |
|---|---|---|
| SAE cold-start rating (W) | 5W — better low-temp flow. | 10W — thicker at cold temps. |
| Viscosity @ 100 °C (typical) | ~9.3–12.5 cSt (SAE 30 band). | ~12.5–16.3 cSt (SAE 40 band). |
| HTHS (load film) | Lower vs 40 (but many modern 5W-30 synthetics have competitive HTHS). | Generally higher — better film under load/heat. |
| Cold-start protection | Superior | Adequate in mild cold, worse in severe cold. |
| High-temp / heavy-load protection | Good (depends on formulation) | Better margin for towing/performance/high heat. |
| Fuel economy | Slightly better (lower viscous drag) | Slightly worse (more viscous drag) |
| Ideal climate | Cold → temperate | Temperate → hot |
| Typical users | Modern cars, cold climates, economy-focused drivers | Towing, hot climates, older/high-mileage engines |
| Pros | Easier cold starts, lower drag, meets many modern OEM choices | Stronger film at operating temp, can reduce oil consumption in older engines |
| Cons | Less margin at very high temps / heavy load (unless specially formulated) | Higher viscous losses; worse cold start in very low temps |
(Table based on SAE J300 bands and typical formulation behavior.)
Practical Scenarios
Example 1 — Cold-climate daily driver
- Vehicle: modern petrol compact, daily short commutes, winters −10 to −30 °C.
- Recommendation: 5W-30 (or even 0W-20/0W-30 if OEM allows) — better cold start flow to protect bearings during frequent cold starts. Use a synthetic that meets OEM API/ILSAC spec.
Example 2 — High-mileage older vehicle
- Vehicle: 10+ years, modest oil consumption, lives in a moderate to hot climate.
- Recommendation: 10W-40 (or sometimes 5W-40) — thicker operating viscosity helps reestablish a stronger oil film in worn clearances and can reduce consumption and noise. Use a high-detergent additive package and monitor consumption. Verify the product meets necessary specs.
Example 3 — Performance car / high-heat environment
- Vehicle: turbo, spirited driving/track, ambient temps frequently >30–40 °C.
- Recommendation: 10W-40 (or manufacturer-recommended 5W-40/0W-40 performance oil) — higher operating viscosity and robust HTHS help resist shear/film breakdown under sustained high loads. If OEM calls for 5W-30 but you do track days, consider a track-focused oil that meets the engine’s HTHS and wear specs or follow OEM guidance for track use (some OEMs require different oils for track).
Expert Recommendation & Conclusion
- Start with the OEM manual. It’s the baseline for warranty and optimal engine longevity.
- Match both viscosity and performance spec (API/ACEA/ILSAC) — not just one or the other.
- If you need all-season, cold-start performance and OEM permits — choose 5W-30 (or lower W). It reduces start-up wear and improves economy.
- If you regularly tow, drive hard, live in hot climates, or have an older engine with higher clearances — choose 10W-40 (or a high-quality 40-grade) to get more film thickness at operating temperature.
- Quality over number. A well-formulated synthetic 5W-30 with high HTHS and good additive chemistry can outperform a marginal 10W-40. Always pick a reputable brand and the correct API/ACEA label.
