Engine oil is one of those things that most drivers know they should top up but aren’t always sure how to choose correctly. The wrong oil won’t necessarily destroy your engine immediately, but using the right grade consistently makes a real difference to engine longevity. Here’s a straightforward guide to understanding what your car needs and why.
Why Oil Grade Matters
Engine oil serves multiple purposes: it lubricates moving parts, reduces friction, helps cool the engine, cleans internal components, and provides a protective film that prevents metal-to-metal contact. Different engines are built to different tolerances and operate under different conditions, which is why there’s no single oil that suits all cars.
Using an oil that’s too thick can make the engine harder to start in cold weather and may not circulate quickly enough on startup — the period when most engine wear occurs. Using an oil that’s too thin may not provide adequate protection at high operating temperatures. Getting it right matters.
Understanding the Numbers
Engine oil grades are expressed as a combination of two numbers separated by a W — for example, 5W-30 or 10W-40. The W stands for Winter, and the number before it indicates the oil’s viscosity (thickness) in cold conditions. The lower the number, the better it flows in cold weather. The number after the W indicates viscosity at normal operating temperature.
So a 5W-30 oil flows more easily on a cold start than a 10W-40, and is thinner at operating temperature too. Modern engines — particularly those with stop-start systems or variable valve timing — are often designed around thinner oils like 5W-30 or even 0W-20, which reach all parts of the engine more quickly on startup.
Older engines, particularly those with higher mileage or looser tolerances, may benefit from a slightly thicker grade like 10W-40, which can help maintain adequate oil pressure as internal clearances increase with wear.
Synthetic, Semi-Synthetic, or Mineral?
Beyond the grade, oil comes in three basic formulations.
Fully synthetic oil is produced through a chemical process that creates highly uniform molecules. It provides better performance across a wider temperature range, lasts longer between changes, and offers superior protection — particularly on cold starts and at high temperatures. Most modern cars specify fully synthetic oil.
Semi-synthetic (or part-synthetic) is a blend of synthetic and mineral base oils. It offers better performance than straight mineral oil at a lower cost than full synthetic. It’s suitable for many older and less demanding engines.
Mineral oil is refined directly from crude oil and is the most basic formulation. It’s suitable for older, simpler engines and is typically changed more frequently. Most modern cars don’t specify mineral oil.
The general rule is to use whatever the manufacturer specifies — typically full synthetic for anything made in the last 15–20 years.
How to Find the Right Oil for Your Car
The definitive answer is in your vehicle handbook. Every manufacturer specifies the oil grade and specification required, and this should be your first reference. If you don’t have the handbook, the information is usually available on the oil filler cap or a sticker under the bonnet, and most oil brand websites and motor factor apps have a vehicle lookup tool — enter your registration and they’ll tell you exactly which oil is correct.
Don’t rely on what the previous owner used, what the garage recommends without checking, or what’s on offer. Match the specification the manufacturer requires.
Manufacturer Specifications
Beyond the grade, some manufacturers have specific oil approval standards. Volkswagen Group cars often require oils meeting VW 504.00, VW 507.00, or similar designations. BMW has its Longlife specifications (LL-01, LL-04). Mercedes uses MB 229.x specifications. These aren’t just marketing — they denote oils tested and approved to specific standards by the manufacturer.
Using an oil of the correct grade but without the required manufacturer approval isn’t necessarily catastrophic, but it’s not ideal, and it can affect warranty coverage on newer vehicles. Look for the approval marks on the oil bottle.
How Much Oil Does Your Car Take?
Check the dipstick — it has minimum and maximum marks, and you want the level between them, ideally closer to the maximum. Don’t overfill; excess oil can pressurise the engine and cause leaks or other issues. When topping up, add small amounts at a time (100–200ml), wait a moment, and recheck before adding more. A full oil change on most cars takes between 4 and 6 litres, but a top-up between services typically requires much less.
How Often Should Oil Be Changed?
Follow the manufacturer’s service interval — typically every 10,000–15,000 miles or 12 months for modern cars on full synthetic oil, whichever comes first. Some manufacturers have extended service intervals of up to 20,000 miles on specific oils. Cars used predominantly for short journeys may benefit from more frequent changes, as short trips don’t allow the engine to reach full operating temperature consistently, leading to moisture and contaminant build-up in the oil.
Never extend oil change intervals beyond the manufacturer’s recommendation to save money. Oil changes are among the cheapest forms of engine maintenance; engine damage from neglected oil changes is among the most expensive.
What If You’re Not Sure?
When in doubt, check the handbook, use the oil brand’s vehicle lookup tool, or ask at a reputable motor factor or franchise dealer. A few minutes of checking is worth it. Getting oil grades wrong isn’t an emergency, but it’s also a very easy thing to get right with minimal effort.
Frequently Asked Questions
How do I know which oil my car needs?
Check your vehicle handbook — this is the definitive source. You can also check the oil filler cap, look for a sticker under the bonnet, or use the vehicle lookup tool on any major oil brand’s website.
What does 5W-30 mean on engine oil?
The number before the W (Winter) indicates how the oil flows in cold conditions — lower is better in cold weather. The number after indicates viscosity at normal operating temperature. 5W-30 is a common specification for modern petrol engines.
Can I mix different engine oils?
You can mix oils of the same grade in an emergency, but it’s not ideal. Never mix different viscosity grades or synthetic and mineral oil as a regular practice. Top up properly at the next opportunity.
How often should I change my engine oil?
Follow the manufacturer’s service interval — typically every 10,000–15,000 miles or 12 months for modern cars on full synthetic oil, whichever comes first. Cars used mainly for short journeys may benefit from more frequent changes.
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