0W-20, 5W-30 Or 5W-40? What The Numbers Actually Mean

The two numbers measure two different things

A grade like 5W-30 is not one measurement written twice. It is two separate tests, at two very different temperatures, reported side by side. The standard behind it is SAE J300.

The first number, the one with the W, is about cold. It is set by two low temperature tests: a maximum cold cranking viscosity, which asks whether the starter motor can turn the engine over, and a maximum low temperature pumping viscosity, which asks whether the oil pump can actually move the oil once it has.

That second test exists because of a real failure mode. Engines can start on oil that is too thick to circulate, and an engine running with no oil reaching the top end does not stay an engine for very long.

The second number is about hot. It is a kinematic viscosity band measured at 100°C, which is far closer to the temperature your oil actually lives at.

So 5W-30 does not mean "5 when cold and 30 when hot." It means this oil passed the 5W cold tests and lands in the 30 band at 100°C. Different scales, different questions.

The winter number is tested at temperatures we never see

Here is where the local conversation usually goes wrong, so let us put the actual figures down.

The Meteorological Service Singapore records overnight minimums that usually do not fall below 23°C to 25°C. The gap between a Singapore morning and a 0W pumping test is more than sixty degrees.

What that means, stated carefully: for a car that lives its whole life here, the winter number is rarely the thing separating one suitable oil from another. It is not a claim that 0W is pointless, and anyone who tells you flatly that the W number is irrelevant in the tropics is going beyond what the standard actually says. Manufacturers specify a winter grade for their own engineering reasons, including how quickly oil reaches the top of the engine on a start from any temperature.

The honest version is this. The winter number is not usually your decision here. The next two sections are.

The number that is not printed on the bottle

SAE J300 sets one more limit, and it is the one that matters most for protection under load. High temperature high shear viscosity, usually written HTHS, is measured at 150°C under an extreme shear rate of around one million reciprocal seconds. It simulates what the oil is doing while it is being squeezed through a loaded bearing at operating temperature.

These are minimums, not values, and that is the useful part. Two oils can both be 5W-40 and still be meaningfully different products. Motul's own data sheets give its 8100 X-cess 5W-40 an HTHS of 3.7 mPa·s, while its 300V Competition 5W-40, a racing oil, sits at 4.0.

Same three characters on the front of the bottle. Different oil.

Approvals are the part that actually decides

Somewhere in your handbook, usually in small type next to the viscosity grade, there is a code. It looks like one of these:

• MB 229.5 or MB 229.51, from Mercedes-Benz
• BMW Longlife-01, LL-04, LL-17 FE+
• VW 502 00, 504 00, 505 00, 507 00
• Porsche C20, C30, A40
• dexos, from General Motors

That code is not marketing. It is a specification the oil has to be tested against and formally listed on, and the manufacturers publish the lists.

Mercedes-Benz runs a public register of approved products by sheet number. Open the sheet for 229.5 and you get the grades it covers, 0W-30, 0W-40, 5W-30 and 5W-40, and the actual product names that hold the approval. If a bottle in front of you claims MB 229.5, you can go and check whether it is on the list. Volkswagen publishes its oil quality standards the same way, with a list of conforming oils by brand and grade.

There is a second thing worth learning to read. Oil manufacturers themselves distinguish between an oil that is approved to a specification and one that is merely recommended for it, or that "meets or exceeds" it. Castrol, for example, lists these as two separate blocks on the same product page. Approved means the OEM tested and listed it. Recommended is the blender's own view.

Both blocks are legitimate information. They are not the same claim, and the label is where the difference is hiding.

API, ACEA and the marks on the bottle

API, the American system

The API "donut" on the back of the bottle tells you three things: the performance level, the SAE viscosity grade, and whether the oil carries a Resource Conserving classification.

The current petrol category is API SQ, introduced in March 2025. API describes it as designed to provide "protection against both fresh and aged oil low-speed pre-ignition (LSPI), improved timing chain wear protection, improved high temperature deposit protection for pistons and turbochargers, sludge and varnish control."

API SP is still current and described as being for 2025 and older engines, and older categories remain listed by vehicle age: SN for 2020 and older, SM for 2010 and older, SL for 2004 and older. Anything from SH backwards is obsolete, and API attaches an explicit caution against using those in modern engines.

There are two other marks worth recognising. The starburst means the oil meets ILSAC GF-7A, and API states it cannot be applied to 0W-16 oils. The shield means GF-7B and can only be applied to 0W-16. If you run a 0W-16 hybrid, that distinction is yours specifically. One more date to know: API lists ILSAC GF-6A as going obsolete on 1 October 2026.

ACEA, the European system

ACEA splits its light duty sequences into two families. The A and B categories are what ACEA calls high SAPS. The C categories it describes as "Catalyst and GPF/DPF compatible engine oils for gasoline and diesel engines, low SAPS."

SAPS stands for sulphated ash, phosphorus and sulphur. Those compounds do useful work in the oil, and they also leave residue that accumulates in particulate filters and catalytic converters. In ACEA's own limits, the A and B categories allow sulphated ash between 1.0% and 1.6% by mass, while the C categories cap it at 0.8% or lower.

ACEA's stated reason for the C sequences is direct: "These oils will increase the DPF/GPF and TWC life and maintain the vehicle's fuel economy." It also adds a warning worth repeating, that some categories "may be unsuitable for use in certain engine types, consult the manufacturer's owner manual/handbook in case of doubt."

Within the C family the film strength differs too. C3 requires HTHS of at least 3.5 mPa·s, C2 at least 2.9. Two low SAPS oils, two quite different specifications. And note that ACEA A3/B3 and C1 were withdrawn with the current sequences, so an oil still advertising them is working from an old sheet.

If your car has a petrol or diesel particulate filter and you put a high SAPS oil through it, nothing dramatic happens on the day. It is a slow, expensive kind of wrong.

Synthetic, semi-synthetic, mineral

These words do less work than the marketing implies, but they do have a technical basis. API defines five base oil groups in API 1509.

Groups I and II come from refining crude. Group III is heavily hydroprocessed and is what most oils sold as full synthetic are built on. Group IV and V are chemically constructed.

Two things follow. First, "full synthetic" is a description of the base stock, and the finished oil is base stock plus an additive package that does much of the actual work. Second, and more usefully: an oil that carries the approval your engine needs has already demonstrated performance in the tests that approval requires, whatever it is made from. The approval is evidence. The word on the front of the bottle is a category.

Why thicker is not automatically safer

This is the most common piece of well meant bad advice in Singapore. The car is in a hot climate, so surely a heavier oil gives a stronger film.

Three reasons to be careful.

Excess viscosity costs energy. Thicker oil means more drag in the bearings and moving parts. Thinner oils, as Machinery Lubrication puts it, "slip through the bearings easily and offer a fuel economy benefit, whilst still protecting against wear." The cost of going heavier than specified is real, and there is no free protection in it.

Modern engines use oil as a hydraulic fluid. Variable valve timing systems actuate cam phasers using oil pressure, so oil outside the specified grade changes how those systems behave, not just how well things are lubricated. This is a well established mechanism in the trade literature rather than an OEM statement, so treat it as a reason for caution rather than a specification.

The specification already accounts for the heat. The grade in your handbook was chosen against clearances, oil pump capacity, gallery sizes and the temperatures the engine was validated at. Manufacturers sell the same engines into hot markets. They did not forget.

There is even a documented fuel economy element inside the approvals themselves. Motul notes on its own data sheet that MB 229.5 "requests fuel economy performance: 1.7% fuel economy improvement versus a 15W-40 reference." Viscosity is a trade off that the specification has already balanced.

If your engine is consuming oil and someone has suggested a heavier grade as the fix, that is worth a conversation rather than a purchase. The consumption has a cause.

LSPI, and why small turbo engines are fussy

If your car has a small capacity turbocharged direct injection petrol engine, which covers a large share of what is sold here now, there is a specific reason your oil choice is not casual.

Low speed pre-ignition is described by Infineum, a major additive manufacturer, as "an abnormal combustion event" characterised by "spontaneous ignition of the fuel that starts before the spark is generated," affecting "smaller, turbocharged, gasoline direct injected engines." The worst case damage it lists is not subtle: "damaged pistons, broken piston rings and bent connecting rods," plus "cylinder head failures and cracked piston lands and skirts."

Mobil describes the same phenomenon as creating "excessive pressure within the engine's cylinders, which can cause permanent damage to cylinder walls, pistons, piston rings, connecting rods and spark plugs."

Here is the part that connects it back to the label. LSPI frequency is sensitive to how the oil is formulated, in particular the detergent chemistry, with published SAE work on reducing it by substituting magnesium detergent for calcium. That is not something you can see, smell or infer from a viscosity grade. It is the reason the modern API and ACEA categories exist, and the reason "it is the right grade" is not the same as "it is the right oil."

How to get this right in about two minutes

1. Open the bonnet and read the oil filler cap. Many manufacturers print the viscosity grade right on it. That is your starting point, not your finishing point.
2. Find the maintenance section of the handbook. You are looking for two things printed together: the SAE grade, and any specification or approval code such as MB 229.5, VW 504 00, BMW LL-04, Porsche C30, dexos or an ACEA sequence like C3.
3. Match both. The grade alone is not the specification. If your book asks for an ACEA C sequence, an A3/B4 oil of the same grade is the wrong oil regardless of price or brand.
4. Check the claim if you want to. For Mercedes and Volkswagen approvals in particular, the approved product lists are public. You can look up whether a given oil actually holds the approval printed on it.
5. Ask what is going in before it goes in. Any workshop should be able to tell you the exact product name and grade, and show you the bottle.

We stock Castrol, Motul and Liqui Moly, and our engine oil packages start from $85 including four litres, the oil filter, a 29-point check and labour. The full list is on our servicing page. Tell us the car and we will match the grade and the approval your handbook asks for. If your handbook asks for something we do not carry, we will tell you that too, rather than putting the closest thing in.

Common questions

Key takeaways

Sources

• SAE InternationalJ300 Engine Oil Viscosity ClassificationThe standard that defines the SAE viscosity grades, the low temperature cranking and pumping limits for each winter grade, the kinematic viscosity bands at 100°C and the minimum HTHS at 150°C. The standard itself is a paid publication.
• American Petroleum InstituteAPI's Motor Oil GuideAPI's consumer guide, containing the current and obsolete service category tables, the description of API SQ, and what the donut, starburst and shield marks mean, including the 0W-16 restriction.
• American Petroleum InstituteAPI 1509, Engine Oil Licensing and Certification SystemThe standard itself. Annex E sets out the base oil group definitions from Group I to Group V, with the saturates, sulfur and viscosity index limits for each.
• ACEAACEA Oil Sequences for Light-Duty EnginesThe European specification. Defines the A/B and C categories, expands SAPS, sets the chemical limits including sulphated ash, and gives the HTHS requirements for C2 and C3.
• Mercedes-BenzOperating Fluids, Sheet 229.5The manufacturer's public approvals register, listing the viscosity grades the sheet covers and every product currently approved against it by name.
• VolkswagenTechnical Bulletin, Volkswagen Engine Oil Quality StandardsAn official VW service bulletin enumerating the VW oil quality standards including 502 00, 504 00, 505 00 and 507 00, with an attached list of conforming oils by brand and grade.
• InfineumControlling low-speed pre-ignitionAn additive manufacturer's technical article defining LSPI as an abnormal combustion event in small turbocharged direct injection petrol engines, and listing the specific hardware damage it causes.
• Machinery LubricationYes, I do understand oil viscosity, or do you?A technical explanation of what the two low temperature tests behind the W grade actually ask, and of the energy cost of running more viscosity than an engine needs.