In short: choose between an oil 5W40 Or 10W40 plays a direct role in engine protection, especially when starting cold and during extended journeys. The 5W40, more fluid at low temperatures, favors cold weather starts and newer engines, while the thicker 10W40 is better suited to temperate climates and older blocks. The maintenance manual remains the reference, but the climate, the age of the vehicle and the type of journey (city, highway, heavy loads) also guide the right choice. In summary: 5W40 for severe winters and modern engines, 10W40 for milder regions and engines that already have mileage.
Key points to remember:
- 5W40 : very fluid when cold, ideal in cold regions and for recent engines.
- 10W40 : thicker when cold, suitable for temperate climates and older engines.
- The two share a clue 40 hot, with protection comparable to operating temperature.
- THE climate, theengine age and the driving habits must be taken into account.
- THE manufacturer's manual remains the reference to respect to avoid premature wear.
5W40 or 10W40 engine oil viscosity: what do these numbers really mean?
Okay, let's see this together in a simple way. The mentions 5W40 And 10W40 are not mysterious codes reserved for pros. They describe the viscosity of oil, that is to say its ability to flow and form a protective film around moving parts. The more viscous the oil, the thicker it is. The more fluid it is, the more quickly it circulates, especially when cold.
The first number, 5 Or 10, corresponds to the behavior of the oil at low temperatures. The letter W means Winter. An oil 5W remains more fluid in the cold than a 10W. Result: in winter or during a morning start, the 5W40 circulates faster in the engine. This is when the mechanics suffer the most, because the metal surfaces are still “dry” and require an instant oil film.
The second number, 40, describes the hot viscosity, when the engine is running at its normal temperature, around 90 to 100°C. A 5W40 and a 10W40 therefore share the same thickness of protective film once everything is hot. This is why the difference between these two grades is mainly played out at startup and in cold regions.
Imagine a driver like Marc, who lives in the mountains and starts his diesel at -10°C in the morning. With oil that is too thick, the oil pump struggles to send the lubricant everywhere. With a 5W40, more fluid, the engine receives its protection more quickly. Conversely, an older city car that drives mainly in town in the south of France can run very well with 10W40, without any stress, as long as the manufacturer's recommendations are respected.
In practice, viscosity is therefore used to adapt the oil to the climate and the type of engine, to keep a balance between fluidity and protection. It’s this compromise that makes the difference between an engine that ages well and a block that starts to click too early.
Comparison table 5W40 vs 10W40 depending on temperature
To clarify the difference in behavior at different temperatures, a table is worth a thousand words.
| Oil index | Behavior at -30°C | Behavior at -10°C | Behavior at 100°C |
|---|---|---|---|
| 5W40 | Very good fluidity, rapid circulation from the start | Good fluidity, easy starting and limited wear | Stable viscosity, protective film rated 40 |
| 10W40 | Slower circulation, more demanding starting | Fluid but thicker than a 5W, “greasy” protective film | Stable viscosity, same level of protection as a 5W40 when hot |
This table clearly shows that the 5W40 takes the advantage as soon as the thermometer drops. The 10W40 however remains a serious candidate in mild climates, especially for engines with a little wear which benefit from a thicker film.
5W40 or 10W40 oil: impact on engine protection and longevity
The question that often comes up in the garage is simple: can oil really change the lifespan of an engine? The answer is yes. Oil is not just for greasing. She lubricates, cools, cleans and protects from corrosion. A poor choice of index can accelerate wear, especially on engines that are heavily used or already marked by kilometers.
An oil 5W40 starts working immediately when cold. The oil film forms quickly between the crankshaft, pistons and camshafts. On a modern engine with low mechanical play, this rapid reaction reduces friction and therefore the risk of scratches or micro-seizing. In the long run, this results in an engine that maintains good compression and controlled oil consumption.
There 10W40, a little thicker when cold, offers a very protective film once the engine is hot. On an older vehicle, which has already gained some play, this additional thickness often helps limit clicking noises and slightly reduce oil consumption. Frankly, for certain sedans from the 2000s that are still driven a lot, this strategy works very well.
Another key point in 2026: many 5W40 and 10W40 oils are now synthetic or semi-synthetic. They resist oxidation better and keep their original viscosity for longer. Result: drain intervals become longer, provided you follow the manufacturer's recommendations and check the level regularly. An oil that is too old, even if well chosen initially, loses its protective capacity.
A little mechanic's advice: if an engine starts to “click” when cold, with a metallic noise a few seconds after starting, the cause may be poorly adapted or too tired oil. Going from a 10W40 to a 5W40 in a cold region, or carrying out an early oil change, is sometimes enough to make these noises disappear. The main thing is to never drive for a long time with an oil warning light on, otherwise serious damage will result.
Specific advantages of 5W40 and 10W40 oils
To help you decide, here is a clear summary of the concrete benefits of each grade, in everyday life.
- 5W40 oil : excellent cold starts, immediate protection of moving parts, good help in reducing fuel consumption, particularly suitable for modern gasoline and diesel engines, including with turbo.
- 10W40 oil : often more affordable price, slightly thicker oil film at steady state, very appreciated on older engines or with high mileage, especially in temperate or hot climates.
In summary, the 5W40 plays the card of cold comfort and efficiency on recent engines. 10W40 remains a safe bet for extending the life of older cars, provided you do not drive them constantly at -15°C.
Climate, season and driving style: how to adapt 5W40 or 10W40
The choice between 5W40 And 10W40 does not only depend on the car model. The region where the vehicle drives weighs heavily. Between a motorist who lives in Lille with wet and sometimes cold winters, and another who drives all year round on the Côte d'Azur, the need is not the same.
In cold areas, where the thermometer can drop below -10°C, an oil 5W40 appears clearly more suitable. The oil pump forces less, starting is smoother, and the lubricating film reaches sensitive areas like the turbo or camshaft bearings more quickly. On a modern SUV often used for short commutes, this choice makes a real difference in long-term wear and tear.
Conversely, in regions with a temperate or hot climate, the 10W40 holds its place perfectly. For a fifteen-year-old gasoline minivan that travels on highways and departmental roads, this higher viscosity when cold is not a problem. Once hot, the engine benefits from a well-adhered film, which can reassure a mechanic that has already run well.
Driving style also comes into play. Urban use, with lots of starting and stopping, accentuates the cold operating phases. In this case, a more responsive 5W40 limits repeated friction and slightly supports fuel consumption. For a vehicle that mainly makes long journeys at steady speed, the difference between 5W40 and 10W40 becomes less visible, as long as the climate remains mild.
A driver like Sophie, who only covers short trips of 5 km in winter, has every interest in switching to a 5W40 if authorized by the manufacturer. On the other hand, Alain, who travels several hundred kilometers every weekend with his 250,000 km diesel station wagon, will benefit from a 10W40 well controlled, checked regularly to avoid driving without oil.
Factors to analyze before choosing your engine oil
To avoid making a mistake, four main parameters should be examined before filling the crankcase.
- Average temperature of the region : harsh or mild winters, altitude, frequent or not periods of frost.
- Type of engine : gasoline, diesel, turbo, direct injection, modern or older engine.
- Vehicle use : short city trips, long highway trips, towing or heavy load.
- Manufacturer recommendations : authorized viscosity indices, required approval standards, oil change intervals.
By combining these elements with the instructions in the manual, the choice between 5W40 and 10W40 quickly becomes clear, without unnecessary headache.
Manufacturer's recommendations: maintenance manual and 5W40 / 10W40 compatibility
Don't panic, it's simpler than it seems: the best compass remains the maintenance manual. Manufacturers indicate the viscosity indices compatible with the engine according to external temperatures. There is often a table with several possible grades: for example 5W30, 5W40 Or 10W40, each associated with a temperature range.
If the manual allows both 5W40 And 10W40, the driver can adapt his choice to the season and the condition of the vehicle. Some opt for a smart compromise: 5W40 for winter, 10W40 for summer, especially when the car has high mileage. This strategy remains valid as long as the mentioned approval standards, for example ACEA or API, are respected.
On recent engines, particularly those with particle filters or complex pollution control systems, you must remain vigilant. Even if the viscosity “seems” compatible, only an oil meeting exactly the brand’s specifications guarantees proper operation and avoids engine lights on or premature clogging. A poorly chosen grade can cause unwanted noise, excess consumption or repeated starting difficulties.
Serious maintenance monitoring also involves regular checking of the oil level and by oil changes carried out on time. In severe use, such as intensive urban driving or towing, slightly shortening the recommended interval allows you to keep the oil clean and effective, whether it is 5W40 or 10W40.
Summary table: engine type, climate and recommended viscosity
This summary gives an overview of the most common combinations, without replacing the manufacturer's official recommendations.
| Engine type | Privileged climate | Often recommended viscosity | Practical notes |
|---|---|---|---|
| Modern gasoline or diesel engine | Cold to temperate climate | 5W40 | Protects very well at startup, suitable for modern systems |
| Old or high mileage engine | Temperate to warm climate | 10W40 | Thicker film, useful to compensate for internal wear |
| High performance engine | Variable depending on use | According to manufacturer's instructions | Specific oils often required, strictly follow the manual |
This table provides a guideline, but the golden rule remains simple: if the manual imposes a specific grade, there is no point in trying exotic experiences. Compatibility comes first.
Differences 5W40 / 10W40 on a daily basis: performance, consumption and driving comfort
Once on the road, what does the passage of a car really change? 5W40 to a 10W40 or the opposite? On certain vehicles, the difference is felt from the first start. With a 5W40 in cold weather, the engine revs more easily, the idle stabilizes more quickly and mechanical noises are often less present during the first minutes.
During sustained driving, on the highway for example, the two oils display a index 40 hot, therefore a similar theoretical level of protection. However, on a high-mileage engine, some motorists feel a slight improvement in acoustic comfort with a 10W40, the thicker film better damping internal clearances. Frankly, for a family that has lived well, this little extra can be appreciated.
In terms of consumption, 5W40 maintains the advantage over cold phases. Less friction when starting means an engine that forces a little less and therefore slightly reduced consumption, especially if the journeys are short and repeated. The difference is not spectacular, but over the year, it can be felt in the fuel budget.
On an environmental level, a well-chosen oil also limits polluting emissions. Good lubrication improves combustion and reduces unburned fuel. An engine that runs easily when cold with a 5W40 will often emit less white or bluish smoke than a unit that struggles to heat up with oil that is too thick for the climate.
In summary, the correct oil index, 5W40 or 10W40, is not only seen on the label of the can. It is felt every day in the flexibility of the mechanics, the noise felt on board, the regularity of starts and the general condition of the engine over the years.
What is the main difference between 5W40 oil and 10W40 oil?
The major difference concerns cold viscosity. A 5W40 oil is more fluid at low temperatures than a 10W40. It therefore circulates more quickly when starting and protects the internal parts of the engine more quickly. When hot, the two oils share an index of 40, which means they offer a comparable level of protection once the engine is up to operating temperature.
Should we favor 5W40 in winter and 10W40 in summer?
In many cases, yes, provided the vehicle manual allows both grades. 5W40 is particularly suitable for cold winters thanks to its very good fluidity at low temperatures. 10W40 can be used in hot seasons or in temperate climates, especially for older engines. However, certain engines require a specific type of oil all year round: the manufacturer's recommendations remain priority.
Is 10W40 oil suitable for a recent engine?
Some recent engines still accept 10W40, but many now require thinner or lower ash oils, such as specific 5W30 or 5W40. Before filling the crankcase, it is essential to check the standards indicated in the maintenance manual. Using oil that is too thick or not approved can disrupt the pollution control systems and accelerate the wear of certain parts.
Can we mix 5W40 and 10W40 in the same engine?
In case of occasional top-up, mixing a small volume of 5W40 and 10W40 does not cause immediate breakage, but it is not a good habit. The mixture modifies the properties of the oil and may deviate from the specifications provided by the manufacturer. In practice, it is better to top up with the same grade as that already present, then carry out a complete oil change as soon as possible if the oils used are different.
How often should you change 5W40 or 10W40 oil?
The oil change frequency depends on the type of engine, the oil chosen and the conditions of use. Many manufacturers place the interval between 15,000 and 30,000 kilometers, or every one to two years. In intensive urban use or for an old vehicle, it is often wise to shorten these times a little. In all cases, the maintenance manual provides a precise schedule that is best followed to preserve the longevity of the engine.