Engine Oil Explained: Types, Grades, and How to Choose the Right One

Engine oil is the lubricant coating the metal parts inside your engine. It cuts friction, carries heat away from the hottest points, keeps the engine running smoothly, helps fuel economy and extends the life of the car. Picking the right grade and changing it on schedule is basic maintenance that every driver ought to understand, because letting oil degrade turns a few hundred baht of servicing into a repair bill in the tens of thousands.

This guide covers everything worth knowing in one place: the types and how they differ, how to read the numbers on the bottle, choosing the right oil for your vehicle, change intervals, what the colour on the dipstick is telling you, and the shelf-life question most people never think about. And if you are shopping for oil, filters, accessories or looking at a used car, Talata brings listings from many sellers together so you can compare prices and contact them directly.

What engine oil is and what it does

An engine runs on metal parts sliding against each other at high speed and high temperature. Engine oil slips between those surfaces as a thin film so they neither wear away nor build up damaging heat. Picture it as a slippery layer doing two jobs at once: keeping metal off metal, and carrying heat out of the hottest part of the engine.

It does considerably more than just lubricate, though:

• Lubricates, reducing friction between pistons, rings, crankshaft and every other moving part
• Cools, drawing heat away from the combustion area so the engine does not overheat
• Cleans, picking up soot and combustion deposits and carrying them to the filter
• Protects against rust and corrosion, coating metal so it does not react with moisture and acids
• Seals, filling the small gaps between piston and cylinder wall so compression does not escape

Over time the additives in the oil are used up, the oil thins and dirties, and both its lubricating and cooling ability drop off. That is why it has to be drained and replaced on schedule rather than simply topped up.

How many types of engine oil are there?

Oil sold in shops and service centres splits into three main types by how it is made, and that directly drives both price and service life.

Mineral oil, sometimes called conventional, is refined straight from petroleum. It is the cheapest and degrades fastest, lasting roughly 3,000 to 5,000 kilometres. It suits older cars and light use where the budget matters most.

Semi-synthetic blends mineral oil with synthetic. You get mid-range quality at a mid-range price and roughly 5,000 to 7,000 kilometres of service. It is the popular choice for ordinary daily driving.

Fully synthetic oil is engineered for uniform molecules, so it handles heat and holds its viscosity better than anything else, lasting roughly 7,000 to 10,000 kilometres. It costs the most but looks after the engine best and needs changing less often, which suits new cars, turbocharged engines and regular long-distance driving.

Those intervals are general figures. The real number is whatever the oil manufacturer prints on the bottle and what your owner's manual specifies, because models and driving styles differ.

How to read engine oil grades and standards

The numbers and letters on the bottle — 5W-30, 10W-40 — are not a secret code. They describe the oil directly, and once you can read them you can choose without guessing.

SAE viscosity and the W number

A grade like 5W-30 is the SAE J300 viscosity rating, and it has two halves.

The number before the W, for Winter, describes how the oil behaves in the cold. The lower the number, the better it flows on a cold engine and the faster it reaches the moving parts after starting. SAE J300 sets the lowest temperature at which each grade will still crank an engine:

In Thailand's climate this figure matters far less than it does in a cold country — even 20W has headroom well below freezing. Newer cars designed around thinner oils still gain on starting and fuel economy.

A grade with a W in the middle is a multigrade, meaning it works both cold and hot. That is different from a monograde, written as a single number such as SAE 40, which only controls viscosity when hot. Cars run on multigrades almost universally now; monogrades survive mainly in older engines and agricultural machinery.

The number after the dash, 30 or 40, describes viscosity at normal running temperature of 100°C. Higher means thicker with a heavier film; lower means thinner and freer-flowing. Most engines sit in the 30 to 40 range. Many new models specify 0W-20 or 5W-30 to cut friction and save fuel, while older or looser engines tend to move up slightly to help compression.

API standards for petrol and diesel

Letters such as API SP or API SQ are the quality standards set by the American Petroleum Institute. Petrol engine grades start with S and diesel grades with C, and the further the second letter runs through the alphabet, the newer the standard.

On the petrol side the newest is API SQ, first licensed on 31 March 2025 for vehicles from the 2026 model year onwards. It builds on API SP with more attention to fuel economy, protection against low-speed pre-ignition (LSPI), reduced timing chain wear and internal cleanliness.

The companion standard to API SQ is ILSAC GF-7, which splits into GF-7A covering 0W-20, 5W-20, 0W-30, 5W-30 and 10W-30, and GF-7B for the ultra-thin 0W-16 grade used by certain newer engines. The useful part is that an API SQ oil can replace one specified as API SP or older, as long as you stay in the viscosity grade your manual calls for. If you see a bottle still marked plain API SN, that standard dates from 2010 and you should reach for something newer.

This table shows which of the letters you will see on Thai shelves are current and which are not.

On the diesel side, the high-tier standard in common use is API CK-4, introduced in 2016 and suited to modern common-rail diesels. Older diesels may specify CJ-4 or CI-4, which are still available but older. One to watch out for is API FA-4: it looks similar to CK-4 but is not interchangeable with it and is not backward compatible, so only use it where the manufacturer specifically calls for it.

Other standards worth knowing

Besides API there is the European ACEA system, which many European models specify directly — ACEA C3, for instance, for engines with a diesel particulate filter. Motorcycle riders have one more to check, the JASO standard.

JASO grades motorcycle oil in four steps by the friction it produces in a clutch test, running from least grip to most.

The mistake riders make most often is putting car oil, or an MB-rated oil, into a wet-clutch bike. The clutch then slips: revs climb but the bike does not pull, because those oils carry friction modifiers added for fuel economy — precisely the opposite of what a clutch plate needs. MA1 and MA2 both suit wet clutches; the difference between them is mostly how the clutch feels as you let it out.

Choosing the right engine oil for your vehicle

The simplest rule is to open the owner's manual first, every time. The manufacturer has already specified the viscosity grade and the standard that engine needs, and following it is safer than following advice passed around. Everything else is adjusting for how the vehicle is actually used.

Petrol vs diesel

Petrol and diesel engines burn differently, so the formulations differ. Petrol engines take an oil starting with API S, such as SP or SQ; diesels take one starting with API C, such as CK-4. Modern common-rail diesels are fairly particular about the standard, because their exhaust after-treatment systems are sensitive to certain additives, and the wrong type can affect them.

Motorcycles

Most motorcycles share one oil bath between engine and clutch, so they need oil made specifically for motorcycles carrying a JASO MA2 rating. Car oil should not be poured in. Automatics such as scooters use a different formulation again, so match the label to the type of bike. Anyone about to take delivery of a used motorcycle should change the oil to the correct specification straight away, because there is no telling what the previous owner used.

Older cars vs new cars

New cars are generally designed around thin oils such as 0W-20 or 5W-30 to cut friction and save fuel. Use what the factory specifies, and resist moving to something thicker on the assumption that it protects better.

As a car ages, the thing that matters is engine condition, not the year on the registration book. A well-maintained ten-year-old car that does not burn oil can happily stay on its original grade. A car showing symptoms — white smoke under acceleration, the dipstick dropping unusually fast, a rattly note when cold — will benefit from moving up one step, say from 5W-30 to 10W-40, which helps compression and reduces oil consumption.

For cars twenty years old and beyond, where the engine has genuine wear, 10W-40 or 15W-40 is where most owners end up. Even then, do not jump straight to something as heavy as 20W-50: oil that is too thick is slower to reach the moving parts on start-up, and start-up is already when most engine wear happens.

What happens if you use the wrong grade or the wrong type?

This matters more than people assume, because nothing goes wrong on the day. The damage accumulates quietly until it turns into a large bill.

Run an oil thinner than the engine wants and the film is too thin to keep metal apart when everything is hot. Rings and cylinder walls wear faster, oil consumption climbs and white smoke can follow. Run one that is too thick and the oil is slow to circulate on a cold start, parts go under-lubricated for longer than they should, the oil pump works harder, and fuel economy suffers because the engine is pushing against the viscosity.

Two other mistakes are more clear-cut: wrong engine type and wrong vehicle type. Petrol oil in a modern common-rail diesel can interfere with the exhaust after-treatment system, and car oil in a wet-clutch motorcycle makes the clutch slip from the first time you open the throttle. Neither of those is a case of "it will do" — they simply should not be used.

If you have already put the wrong oil in and not driven far, the safe move is to drain it and fit a new filter. One oil change costs far less than an engine repair.

Comparing popular grades: 5W-30, 10W-40, 15W-40

Only a handful of grades turn up regularly in Thailand. This table shows which suits what.

The question that comes up more often, though, is which to pick between two grades that are nearly the same. This table answers them in pairs.

The deciding rule is always the manual first, then engine age and how the car is used. Moving one step either way on a car with mileage on it is fine; going much further than that affects both wear and fuel economy.

When to change or drain engine oil

Change intervals follow the type of oil: mineral roughly every 3,000 to 5,000 kilometres, semi-synthetic 5,000 to 7,000, fully synthetic 7,000 to 10,000. The rule people forget is to watch distance and time together and act on whichever arrives first. Even on low mileage, oil sitting in an engine beyond six months to a year degrades with age just the same.

Change the oil filter every time you change the oil, or at the very least every second change, because a blocked filter simply lets dirty oil circulate back through the engine. The small extra cost of a filter beats running fresh oil through an old one.

Signs that a change is due include the engine running rougher or noisier, fuel economy dropping off, the dipstick showing dark thick oil or a falling level, and the oil pressure warning light showing amber or red on the dash. If the red light comes on while driving, find somewhere safe to stop and switch off immediately, because it can mean dangerously low oil pressure. A car that will not start but still has electrical power is usually a separate issue — check the battery and starter motor first.

Whether you change it yourself or take it in is down to preference. With the tools and the space, doing it yourself saves labour, but the old oil has to be disposed of properly and the drain plug torqued correctly. A garage or service centre gives you convenience, a check over everything else while it is up, and a service record — which is a clear advantage when you come to sell the car yourself, because a complete service book negotiates better than a car with no evidence of maintenance.

What the colour of your engine oil tells you

Pulling the dipstick and looking at the colour is the quickest health check you can run yourself. Healthy oil should be clear and translucent with no sediment or unusual foam. This table sets out what each appearance means.

One thing to understand is that diesel oil goes black far faster than petrol oil, simply because it produces much more soot. Diesel oil that has gone dark after a few thousand kilometres is not necessarily spent, so read it alongside distance and time. The colour that genuinely demands attention is the milky one, because it usually means water has mixed into the oil — a bigger problem than a due service.

Does engine oil expire, and how long does it keep?

Engine oil has a shelf life even unopened. A sealed bottle stored somewhere dry, out of sunlight and away from heat generally keeps for around four to five years from the date of manufacture, with the date on the packaging as the authority. Once opened, use it within about a year, because exposure to air degrades the additives faster and moisture can work its way in.

Before pouring oil that has been sitting for a while, check two things. First the manufacture and expiry dates printed on the bottle. Second the oil itself: if it is still clear, consistent in colour, not cloudy, not separated and free of sediment at the bottom, it is probably fine. If it has gone cloudy, unusually sticky, or smells burnt, leave it. Degraded oil does not lubricate properly and can do more harm than good.

Engine oil prices and rough change costs

Price tracks type above all else: mineral is cheapest, semi-synthetic sits in the middle, fully synthetic costs the most. The common retail size is a 4-litre bottle for a saloon. Adding the filter and labour at a garage or service centre, a typical saloon change tends to land somewhere between several hundred and around two thousand baht, depending on the oil and the size of the engine.

What moves the price is the brand and formulation, how much oil the particular model takes, and where the work is done — main dealer, independent garage or your own driveway. Before committing, compare the same oil across a few shops and check whether the quoted figure already includes the filter and the labour, so you are comparing like with like. One thing worth scheduling alongside it is the annual vehicle inspection, since one trip is quicker than two.

Why buy engine oil and car parts on Talata

Once you know which grade and standard your car needs, the next step is finding it at a sensible price. Talata carries listings for engine oil, filters and car parts and accessories from many sellers in one place, so you can compare brand, grade and price before deciding, then contact the seller directly. Anyone working through the used car buying process can also use what is in this guide to judge how well a particular car has been looked after before agreeing anything.

On the selling side, parts shops and anyone with spare oil, filters or accessories sitting unused can list them on Talata for free. State the brand, grade, quantity, manufacture date and condition clearly and buyers decide faster. Whichever side you are on, check the date on the bottle and compare a few listings before committing. When you find the one that matches your car's specification, start the conversation on Talata.

Frequently asked questions

How often should engine oil be changed?

It depends on the type. Mineral runs roughly 3,000 to 5,000 kilometres, semi-synthetic 5,000 to 7,000 and fully synthetic 7,000 to 10,000. Watch the calendar too — low mileage but more than six months to a year old still warrants a change. Follow the owner's manual and the bottle above all.

What's the difference between fully synthetic and semi-synthetic?

Fully synthetic is made with uniform molecules, so it copes with heat and holds viscosity better, lasts longer and costs more. Semi-synthetic blends in mineral oil for a lower price at mid-range quality. For ordinary driving on a budget, semi-synthetic is enough; for a new car, a turbo engine or frequent long distances, fully synthetic looks after the engine better.

What is the difference between API SQ and API SP, and can I substitute one?

API SQ is the newer petrol standard, first licensed on 31 March 2025, adding LSPI protection, reduced timing chain wear and better internal cleanliness compared with API SP. You can use SQ in a car whose manual specifies SP or older, provided you match the viscosity grade in the manual. The reverse does not hold — SP should not go into a car specified for SQ.

What is the difference between JASO MA and JASO MB?

Friction. MA and its sub-grades MA1 and MA2 are designed so clutch plates grip, which is what a wet-clutch motorcycle needs when engine and clutch share an oil bath. MB has low friction and is made for automatic scooters where the clutch is housed separately. Put MB into a wet-clutch bike and the clutch slips — the revs rise but the bike does not pull.

What is the difference between API SP and API SN?

API SN dates from 2010 and API SP from 2020, a decade newer. The main difference is that SP adds protection against low-speed pre-ignition, a problem in modern turbocharged direct-injection engines that SN never covered, along with better timing chain wear protection and internal cleanliness. If you see a bottle still marked plain SN on the shelf, choose an SP or SQ instead.

Can I use 5W-30 and 10W-40 interchangeably?

Check the manual first. Broadly, 5W-30 is thinner and suits newer cars built around fuel economy, while 10W-40 is thicker and suits cars with several years of use behind them. Swapping between the two is workable on a car with mileage on it, but a new car specified at 5W-30 from the factory should not be moved to something thicker without a reason.

My engine oil is black — do I need to change it now?

If the dipstick shows thick black oil and you are near the change interval anyway, yes. On a diesel, though, oil darkening quickly is normal because of the extra soot, so read it alongside distance and time. The colour to act on immediately is milky white, which means water has got in — get it inspected straight away.

Can I still use engine oil that has been stored for a long time?

If it is unopened and has been kept dry and out of the sun, four to five years from the manufacture date is the general guide, with the date on the packaging as the authority. Before pouring, check the oil is still clear, consistent in colour, not cloudy, not separated and free of sediment. Once the bottle has been opened, use it within about a year.

How do I check the engine oil before buying a used car?

Pull the dipstick with the engine cold and look at both colour and level. Well-maintained oil should be reasonably clear and sitting within the marks. Milky colour or foam can mean a coolant leak. Very dark, thick oil sitting low may say the previous owner neglected servicing. Either way it is useful information for negotiating, or for deciding against the car.

What should I include when listing engine oil or spare parts on Talata?

Brand, viscosity grade, the API or JASO standard, how much is left, the manufacture date on the bottle and whether it has been opened. The more complete the listing, the faster buyers decide and the fewer follow-up questions you field.