# Engine oil grades explained: what 5W-30 actually means

> Source: https://motorservicing.co/blog/engine-oil-grades-explained/  
> Provider: Motor Servicing (Fuel Fixer Ltd), mobile car servicing across the United Kingdom.  
> Contact: 0333 366 1081 · Updated: 2026-07-31

_Guides · published 2026-06-13_

**In 5W-30, the 5W is the oil's flow behaviour when cold and the 30 is its thickness at operating temperature. Lower first numbers flow better on cold starts. The manufacturer approval printed on the bottle matters more than the brand, because it certifies the oil meets your engine's specific chemical requirements.**

## Reading the numbers

A grade like 5W-30 describes viscosity at two temperatures. The number before the W ("winter") describes cold flow: 0W flows better cold than 5W, which flows better than 10W. The number after describes viscosity at 100°C: a 40 is thicker there than a 30.

So 0W-20 is thin cold and thin hot, typical of modern petrol engines designed for efficiency. 10W-40 is thicker at both ends, typical of older or higher-mileage engines with larger clearances.

## Why it matters more than people think

Most engine wear happens in the first seconds after a cold start, before oil reaches the top of the engine. That is exactly what the W number governs. Using an oil that is too thick when cold delays that, particularly in winter, and on modern engines with variable valve timing it can also stop hydraulic components working correctly.

Too thin at temperature is the opposite problem: reduced film strength where load is highest, and on some engines increased consumption.

## Approvals beat brands

Look for codes like VW 504.00/507.00, BMW LL-04, MB 229.51, Ford WSS-M2C913-D, or ACEA C3. These are not marketing. They certify the oil has passed the manufacturer's own tests for that engine family, including things a viscosity grade does not describe: additive chemistry, ash content, and compatibility with emissions equipment.

**The expensive mistake**Diesel cars with a DPF need low-SAPS oil (low sulphated ash, phosphorus and sulphur). Standard high-ash oil will block the filter faster, and a DPF replacement is a four-figure bill. The correct grade with the wrong approval is still the wrong oil.

## Long-life servicing

Long-life regimes stretch intervals to 18,000 or 20,000 miles using specific oils and a variable service indicator. They work as designed on high-mileage motorway cars. They work considerably less well on short-journey city cars, where fuel dilution and moisture contaminate the oil long before the indicator asks for a change.

If your driving is mostly short trips, a fixed 12-month or 10,000-mile change is a better idea than the indicator, whatever the interval says.

## How often to change it

At minimum, once every 12 months regardless of mileage. Oil degrades with time as well as use: it oxidises, absorbs moisture, and its additive package depletes. A car that does 3,000 miles a year still needs an annual change.

Our [mobile oil change](https://motorservicing.co/services/oil-change/) takes about 45 minutes on your driveway: old oil drained and taken away, correct-specification oil refilled, new filter, new sump washer, and the service light reset where the car supports it.

## Frequently asked

**Can I mix different oil brands?**  
In an emergency top-up, yes, provided the grade and approval match. As a routine practice, no. Additive packages are formulated as a system.

**What happens if I use the wrong oil?**  
Best case, reduced efficiency and faster degradation. Worst case, on a DPF diesel or a modern turbo petrol, expensive damage to emissions equipment or the turbo.

**Do I need synthetic oil?**  
If your manufacturer specifies it, yes, and almost all modern engines do. On older engines, semi-synthetic is often the correct choice rather than a compromise.


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