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How Oil Grades Work
Engine oil grades use the SAE (Society of Automotive Engineers) system. The format is XW-Y where:
- X = cold viscosity (W stands for Winter). Lower number = better flow in cold weather.
- Y = hot viscosity at 100°C. Higher number = thicker oil film at operating temperature.
SAE J300 actually sets two cold limits for each winter grade: a cranking viscosity (how easily the starter can turn the engine) and a pumping viscosity (whether the pump can move the oil at all). For 5W those are −30°C and −35°C; for 0W, −35°C and −40°C; for 10W, −25°C and −30°C. So a 5W-30 must still crank at −30°C and still pump at −35°C.
The second number is a viscosity band measured at 100°C, but the figure that actually governs film strength under load is HTHS at 150°C: a grade 20 oil must be at least 2.6 mPa·s and a grade 30 at least 2.9. That is the real difference between a 0W-20 and a 5W-30, and it is why you cannot treat them as interchangeable.
A common confusion worth clearing up: the viscosity grade does not determine whether an oil is safe for a DPF. Diesel and petrol particulate filters are blocked by non-combustible ash, and ash comes from the oil’s SAPS level (sulphated ash, phosphorus and sulphur) — that is what the ACEA C categories control. ACEA caps sulphated ash at 0.5% for C3 and C4 and 0.8% for C2, C5, C6 and C7. So “5W-30” is not one oil: a 5W-30 ACEA A3/B4 and a 5W-30 ACEA C3 are very different products, and only the low-SAPS one is safe for a car with a particulate filter. Match the specification (ACEA C-class, or an OEM approval such as PSA B71 2312 or STJLR.03.5007), not just the grade.
Common Grades in the UK
| Grade | Cold Flow | Hot Protection | Typical Use |
|---|---|---|---|
| 0W-20 | Excellent (-40°C) | Light | Modern fuel-efficient engines (Toyota, Honda, VW) |
| 0W-30 | Excellent (-40°C) | Medium | Premium European cars (BMW, Mercedes, Volvo) |
| 5W-30 | Very good (-35°C) | Medium | Most common UK grade — works for most cars |
| 5W-40 | Very good (-35°C) | Higher | Performance/older European engines |
| 10W-40 | Good (-25°C) | Higher | Older engines, semi-synthetic option |
Recommended Engine Oils
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Which Grade Do I Need?
Always use the grade specified in your vehicle handbook. Modern engines are designed around specific oil viscosities — using a different grade can:
- Void your manufacturer warranty
- Increase fuel consumption
- Cause engine wear or damage
- Reduce cold-start protection or fuel economy
Frequently Asked Questions
Can I use 5W-30 instead of 0W-20?
Only if your manufacturer allows it. Many modern engines are designed around 0W-20 for fuel economy and emissions, and the two grades differ in HTHS viscosity at 150C (a grade 20 oil is rated at 2.6 mPa.s minimum, a grade 30 at 2.9), which is what governs film strength under load. Using a heavier grade than specified can increase fuel consumption and slow cold-start oil delivery. It will not, however, light a warning lamp – no mainstream car senses oil viscosity directly. Check your handbook, and match the manufacturer specification as well as the grade.
Is thicker oil better for older engines?
Not necessarily. Stick to the manufacturer’s recommendation. If your engine burns oil, consult a mechanic rather than switching to a thicker grade.
Does the brand of oil matter?
As long as the oil meets the correct grade AND manufacturer specification (e.g., VW 508 00, BMW LL-04), any reputable brand is fine.
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How we check fitment for this guide
We cross-reference fitment figures against manufacturer specifications and OE-supplier catalogue data where available, and matched to the standard codes that actually govern fit — battery type/group sizes, wiper lengths and fitting types, bulb fitment codes, tyre size markings, and oil specifications. We work by make, model, year, engine, and build date, because a single model can take different parts across its life. Where the right part depends on your exact vehicle, we tell you to confirm by registration or VIN. More on our method →