Engine Oil Viscosity Grades
Engine oil is classified by its viscosity — how thick or thin it is at different temperatures. The Society of Automotive Engineers (SAE) grading system uses a two-number format like 5W-30. The first number (with the “W” for winter) indicates cold-start flow, and the second number indicates viscosity at operating temperature (100°C). Lower numbers mean thinner oil.
What the Numbers Mean
5 W — 30
Common SAE Grades Compared
| Grade | Cold Pump Limit (°C) | Viscosity at 100°C (cSt) | Character |
|---|---|---|---|
| 0W-20 | −40 | 5.6–9.3 | Ultra-thin; best cold flow |
| 0W-30 | −40 | 9.3–12.5 | Excellent cold flow; moderate hot protection |
| 5W-20 | −35 | 5.6–9.3 | Thin; good fuel economy |
| 5W-30 | −35 | 9.3–12.5 | Most popular all-round grade in the UK |
| 5W-40 | −35 | 12.5–16.3 | Popular for European cars; thicker hot film |
| 10W-30 | −30 | 9.3–12.5 | Older engine designs; mild climates |
| 10W-40 | −30 | 12.5–16.3 | Common for older/higher-mileage engines |
| 10W-60 | −30 | 21.9–26.1 | High-performance / motorsport; very thick |
| 15W-40 | −25 | 12.5–16.3 | Commercial diesels, older vehicles |
cSt = centistokes (kinematic viscosity). Cold pump limit is the lowest temperature at which the oil can be pumped to the engine.
Common Grades by Car Type
| Vehicle Type | Typical Grade(s) | Why |
|---|---|---|
| Modern Japanese (Toyota, Honda, Mazda) | 0W-20 | Tight tolerances; fuel economy focus |
| Modern Ford / Vauxhall | 5W-20 or 5W-30 | Manufacturer-specified for EcoBoost etc. |
| VW / Audi / Skoda / SEAT | 5W-30 (504/507 spec) | Long-life service intervals; DPF compatible |
| BMW / Mercedes | 5W-30 or 5W-40 | LL-04 or MB 229.51 approvals |
| Older petrol (pre-2010) | 10W-40 | Wider tolerances; thicker oil suits worn engines |
| High-mileage (100k+ miles) | 10W-40 or 5W-40 | Thicker film compensates for wear |
| Performance / track day | 10W-60 or 5W-50 | Maximum protection at sustained high RPM and temperature |
| Van / commercial diesel | 5W-30 or 15W-40 | Heavy-duty protection; manufacturer spec varies |
Seasonal Recommendations for the UK
The UK's climate is relatively mild, so multi-grade oils work year-round for almost all vehicles. A 5W grade is more than sufficient for British winters (rarely below −15°C). There is generally no need to switch oil seasonally unless you are using a classic car in extreme conditions or motorsport applications.
| Season | Temperature Range | Suitable W Grades |
|---|---|---|
| UK Winter | −10°C to +5°C | 0W, 5W (both ideal) |
| UK Spring / Autumn | +5°C to +15°C | 0W, 5W, 10W (all fine) |
| UK Summer | +15°C to +35°C | Any W grade; 10W or 15W acceptable |
Always use the grade specified in your owner's handbook. Using a different viscosity can affect fuel economy, emissions, and engine wear — and may invalidate your warranty.
SAE viscosity grades are defined by SAE International standard J300. Beyond viscosity, engine oils must also meet manufacturer-specific approvals (e.g. VW 504/507, BMW LL-04, MB 229.51). Always check your handbook for the correct specification, not just the viscosity grade.