What Engine Oil Numbers Actually Mean (and How to Find Yours)

• The numbers on an oil bottle, such as 5W-30, show how thick the oil is when the engine is cold (the number before the W) and when it is hot (the number after it).
• The only grade that counts is the one your owner’s handbook lists, and it needs the right ACEA specification too, with diesel cars that have a particulate filter needing a low-SAPS oil.
• A garage oil and filter change typically costs about £100 ($133), and a 1 litre bottle of fully synthetic 5W-30 is listed at £10.57 to £17.99 ($14 to $24) online.

The Two Numbers on Your Oil Bottle Describe How Thick It Is, Cold and Hot

The numbers on a bottle of engine oil, such as 5W-30, describe how thick the oil is. The first number, followed by a W for winter, rates how easily the oil flows when the engine is cold. The second number rates how thick it stays once the engine is at normal running temperature. The right grade for your car is whichever one the manufacturer lists in the owner’s handbook, and a different grade from the shelf is not an upgrade.

The system behind the numbers comes from the Society of Automotive Engineers, and TotalEnergies explains that the W stands for winter. Its UK motorist guide says the first number indicates viscosity at low temperatures, while the second number indicates viscosity at higher temperatures, measured at 100°C. Viscosity is simply resistance to flow. Oil gets thicker as it cools and thinner as it heats, so a multigrade oil is built to behave like a thin oil on a cold morning and a thicker one when the engine is hot.

A lower first number means the oil pours and pumps more easily from a cold start. Motul says a 5W-30 oil flows quickly on cold starts, which reduces strain and wear in the first seconds after the engine fires. A higher second number means the oil holds more of its thickness at running temperature. That is why a 5W-40 and a 5W-30 behave the same on a frosty morning but differ once the engine has warmed up.

How 5W-30 Differs From 5W-40 and 10W-40

The three most common grades are easy to mix up, so the differences are worth knowing. TotalEnergies lists the kinematic viscosity of 5W-30 at 9.3 to 12.5 mm²/s and 5W-40 at 12.5 to 16.3 mm²/s, which makes the 5W-40 the thicker oil when hot. Both are rated to work down to minus 30°C, but the company says the 5W-40 stays effective up to an ambient temperature of 50°C, compared with 30°C for the 5W-30.

TotalEnergies says 5W-40 suits higher-performance engines that run hot or under heavy load, such as urban stop-start traffic or high-speed driving, while 5W-30 suits less demanding use. Against a 10W-40, the 5W-40 flows more easily when cold, so it is better for cold starts, while the company says the two protect about equally against wear once the engine is hot.

Thinner oils also have a fuel economy angle. Motul says lower-viscosity oils such as 5W-30 reduce internal engine resistance and can improve fuel economy without sacrificing protection. Auto Express reports that modern engines generally need thinner oils. Never swap to a thinner or thicker grade for economy or protection alone. The tolerances inside your engine were set for the grade in the handbook.

How to Find the Right Oil for Your Car

Start with the handbook. Autotrader says new cars come with a recommended oil grade, and often a recommended manufacturer, listed in the manual. RAC says to check the owner’s manual for the correct type and grade, and warns that the wrong oil type can damage the engine. If the handbook is missing, Auto Express says your local main dealer can look the car up by registration number and is the most accurate alternative, as the information comes direct from the manufacturer.

Online oil finders are the next best option. Autotrader names tools from Shell, Mobil and Euro Car Parts, and TotalEnergies has its own Lub Advisor. They typically ask for your registration, or the make, model, year, engine and fuel type. Auto Express notes that third-party tools are less accurate than the handbook or a dealer, so treat their answer as a cross-check, not the final word.

The grade is only half the answer, as oil also carries a specification. Autotrader points to ACEA ratings such as A1/B1 and says the brand counts for less than matching both the grade and the ACEA specification, though a reputable maker is a sensible choice. Auto Express explains that petrol oils use ACEA ratings A1, A3 and A5, while diesel oils use B1, B3, B4 and B5. Diesel cars with a diesel particulate filter need a low-SAPS oil rated C1, C2 or C3, so that the filter is not blocked. Some carmakers also use their own specifications, and the handbook can list a more widely available alternative.

Then there is the type of oil. Auto Express says synthetic oils contain fewer impurities and that some modern engines require them, with fully synthetic regarded as the highest performance and semi-synthetic a blend with mineral oil. Mineral oil suits older, simpler engines, including many classics, and is cheaper but degrades faster. Longlife oils, usually fully synthetic, can last up to two years or 18,000 miles, but Auto Express says to check that your car is compliant before using one.

What Happens If You Use the Wrong Grade

A single wrong top-up will not wreck an engine. Autotrader says the wrong oil will not cause immediate damage, but it can cause serious problems and shorten engine life over time. Auto Express goes further on the practical side: wrong oil is better than no oil, and if you can only buy the wrong grade, you should book an oil change as soon as possible. The same article says you can mix brands and grades without severe damage, though using the same oil every time is recommended.

Quantity is a second risk. Autotrader warns that low oil causes friction that can be very costly to repair, and that overfilling is as bad as having a low oil level. If you need to add oil, pour a little at a time and recheck the dipstick before adding more.

How to Check Your Oil Level and When to Change It

Check the level about once a month. Autotrader, RAC and Kwik Fit all give that interval, and RAC adds that high-mileage cars and cars used mostly in town should be checked weekly, as should any car before a long trip. Park on level ground. Autotrader says to check with the engine warm: stop the engine, wait five minutes, wipe the dipstick clean, push it back in fully, pull it out again and read the level. The oil should sit between the two marks. RAC describes the check with the engine off and cool, so follow whichever method your handbook gives. Some cars have no dipstick at all, and RAC says those report the level through dashboard messages.

Look at the colour of the oil too, not just the level. RAC says fresh oil is amber or golden, and that dark brown or black oil that is gritty or thick is likely due a change. Change intervals vary more than most drivers expect. RAC says typically every 5,000 to 7,500 miles, depending on the oil and driving conditions, with short trips, towing and extreme temperatures calling for more frequent changes. Kwik Fit gives 7,500 to 10,000 miles depending on the age of the car and the oil type, and Checkatrade says a newer car on synthetic oil can go up to 10,000 miles while an older car on conventional oil might need changing after 3,000 miles. The manufacturer’s schedule overrides all of them.

If the oil warning light comes on, treat it as an emergency. RAC says to turn the engine off straight away, as driving further, even briefly, can cause severe damage. If the light stays on after you top up, do not drive: call RAC roadside assistance or a garage.

What Oil and an Oil Change Cost

A price comparison listing on Compare A Price shows 1 litre bottles of RAC 5W-30 fully synthetic oil from £10.57 to £17.99, or about $14 to $24 at the current exchange rate of roughly $1.33 to the pound. Those are marketplace listings, so treat them as a guide to the price range rather than a quote. The handbook states how many litres your engine takes, so multiply that by the bottle price to size up the oil bill.

For the whole job, Checkatrade’s 2026 price guide puts an oil and filter change at £25 to £200 ($33 to $267), with about £100 ($133) typical for most cars, and mobile mechanics usually slightly dearer than a garage. Labour typically runs from £40 to £80 an hour ($53 to $107). Autotrader adds a tip: if your car is booked in for a service, you can bring your own oil for the dealer to use, which is worth asking about if you have already bought the right grade.

The simple routine is to read the handbook once, write the grade and specification in your phone notes, and check the level every month. That one habit covers most of what goes wrong with engine oil.

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Sources:

• TotalEnergies: What’s the difference between 5w30 and 5w40 car engine oil ? | Motorist | TotalEnergies UK
• Autotrader: What oil does my car need? | Autotrader
• Auto Express: What’s the best engine oil for my car? | Auto Express
• Motul: Engine Oil 5W30 Guide | Benefits, Applications & Choosing MOTUL Oil
• RAC Drive: How often should you change engine oil? | RAC Drive
• Kwik Fit: How to Check Your Oil Level | Kwik Fit
• Checkatrade: How much does an oil and filter change cost in 2026? UK price guide
• Compare A Price: RAC 5W-30 Fully Synthetic Engine Oil, 1L, Find the Best Prices & Deals UK | Compare A Price
• Pound Sterling Live: British Pound-U.S. Dollar History: 2026