What oil your car needs (and why using any oil won't do)
There’s a question that almost every driver asks themselves sooner or later: what oil does my car need? Sometimes it comes up before a service, sometimes because the oil level warning light comes on, sometimes simply because it comes up in conversation and you realize you don’t know the answer. And the answer that many people give — “whatever they put in at the workshop” — is a reasonable answer in practice, but it leaves out something important: understanding why that particular oil and not another, and what it means to change it at the wrong time or with the wrong product.
You don’t need to become a mechanic. But it’s worth understanding the basics, because engine oil is one of those elements where a small mistake can have disproportionate consequences.
What oil does in the engine
Before we get into specifications, a bit of context. Oil has several functions in an engine: it lubricates the moving parts to reduce friction, helps dissipate the heat generated by combustion, cleans particles and residue that accumulate with use, and protects metal surfaces from corrosion. If the oil is unsuitable for the engine — or is degraded — none of those functions work properly.
An engine running on incorrect or exhausted oil can suffer premature wear, overheating, or damage to parts that are expensive to repair. It’s not catastrophizing: it’s what the manufacturer’s manual says, which designs the engine to run with a specific specification.
Where to find the correct specification for your car
The oil specification is not universal. Each manufacturer, for each engine, defines what type of oil should be used. And that information is available in several places — you don’t have to guess.
The first and most reliable is the owner’s manual. There’s almost always a maintenance or fluids section that shows the recommended viscosity and, in many cases, a specific quality standard (for example, “ACEA C3” or “API SN”). If you have the manual on paper, look in the index under “engine oil” or “lubricants”. If you have it in digital format, it’s a Ctrl+F and done.
The second place is the label under the bonnet. Many cars have a sticker near the oil cap or on the cylinder head cover showing the recommended viscosity. It’s the quickest option when you have the bonnet open.
If you don’t have the manual and there’s no label, the third option is to search on the manufacturer’s website using your car’s Vehicle Identification Number (VIN), which uniquely identifies your vehicle. Many manufacturers have online tools that give you the exact specification for your registration or VIN.
And if all else fails, any trusted mechanic can tell you in thirty seconds with your vehicle details.
What that nomenclature means
Once you have the specification, you’ll find something like this: 5W-30 or 0W-20. It’s not as cryptic as it looks.
The number before the W indicates how the oil behaves in cold temperatures. W stands for winter. The lower that number, the better the oil flows when the engine starts in cold weather — which is when the most wear occurs, because it takes a moment for the oil to circulate through all the passages. A 0W starts better in cold than a 5W, which in turn is better than a 10W.
The number after the hyphen indicates viscosity when hot, when the engine is at working temperature. A 40 is thicker when hot than a 30. Modern engines usually prefer thinner oils when hot (30 or 20) because they’re designed to minimize friction and improve fuel economy.
In short: the first number is particularly important for cold starts and cold climates; the second, when the engine is running at full temperature.
Changing the viscosity from the recommended specification without a clear technical reason is not a good idea. Oil that’s too thick in cold conditions takes longer to reach critical parts. Oil that’s too thin when hot may not provide enough protection under load.
Mineral, semi-synthetic, or synthetic oil
In addition to viscosity, there’s another factor that determines the change interval: the base type of oil.
Mineral oil is refined directly from crude oil. It’s the most economical, but it also degrades faster. In older cars that specify it, it works well, but the change intervals are shorter — typically between 5,000 and 7,500 km, or once a year.
Semi-synthetic oil combines a mineral base with synthetic additives. It’s a middle ground in terms of price and durability, with intervals that typically range around 10,000 km or once a year.
Synthetic oil is manufactured through chemical processes that produce more uniform and stable molecules. It withstands extreme temperatures better, degrades more slowly, and allows for longer change intervals — between 15,000 and 30,000 km in some cases, although always according to what your car’s manufacturer specifies.
Most cars manufactured in the last fifteen years specify synthetic oil. If your car requires synthetic and you put in mineral “because it’s cheaper”, you’re shortening its life, not saving money.
How often to change the oil
The correct answer is: whatever the manufacturer specifies for your specific car, and always whichever comes first between the interval in kilometers and the interval in time.
That last point is important. Oil degrades even if the car doesn’t drive much. If you drive few kilometers a year but the manufacturer specifies an annual change, the annual change still makes sense — the oil has been subject to heating and cooling cycles, moisture condensation, small incomplete combustions. Low kilometers don’t compensate for the passage of time.
In practice, for a modern car with synthetic oil, the most common intervals are between 15,000 and 20,000 km, with a maximum of a year or two depending on the manufacturer. But there are engines — some high-performance diesels, some hybrids — that have different intervals due to their specific combustion or temperature characteristics.
The only way to know the exact interval for your car is to check the manual or the manufacturer’s maintenance plan. And once you know it, it’s worth noting down: because when the time for a change comes, that information isn’t always fresh in your memory.
Why it’s worth recording it
Oil changes are probably the most frequent maintenance operation in a car’s life. And also one of the easiest to lose track of: the workshop records it in their system, sometimes in a service book, but do you know when the last change was and what oil they used?
The data worth keeping from each change is minimal: date, kilometers, oil type, and if possible, the product brand. With that, you know exactly where you are in the cycle at any time, without having to call the workshop to ask. OwnAutoCare is designed to record exactly that — with the receipt attached if you have it — and store it in your Google Drive or iCloud, without intermediate accounts.
And when you sell the car, that detailed history — with the oil, with the services, with the repairs — is exactly what sets a well-maintained vehicle apart from one that makes buyers uncertain.