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.
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 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.