Skip to main content

Engine oils explained

Engine oils explained

We all need to put oil in our engines, but why are there so many types, what are the differences and does it really matter?

Even if your car is relatively new and you take it to a dealer for servicing, you still need to check the oil level. And the older your engine, the more oil it will tend to use (or leak) and the more often you need to check the dipstick to make sure it stays above the minimum level.

If you don’t maintain your car’s minimum oil level, there’s a good chance key engine components will be starved of oil and either wear prematurely or fail catastrophically. That would mean a LOT of expense and trouble – far more than the cost of a litre of oil at the gas station and a dipstick check every weekend.

It’s not just about keeping the oil level topped up. It’s also vital to change the engine oil at the manufacturer’s specified service intervals, because oil wears out. Your engine might be purring like a cat, but inside it the oil is being subjected to constant mechanical and chemical stresses that will slowly degrade its performance.

The mechanical stresses and strains are probably easy to picture and lead to steady molecular breakdown. The chemical degradation comes from the accumulation of contaminants from engine combustion processes and leaky gaskets and oil seals (in older cars), and the effects of oxidation.

Oil isn’t cheap, but it’s a lot cheaper than a premature engine replacement. But maybe it’s not the cost of the oil that’s putting you off but the difficulty of finding the right one?

Buying oil isn’t so simple any more. Modern car engines use extremely fine tolerances and subject oils to a series of very different forces, conditions and stresses. An old car engine might be perfectly happy with a few good glugs of old-fashioned SAE 20W 50 mineral oil, but engine technology has moved on and the demands on engine oils have moved on too. 

So here’s a guide to engine oil types, standards and jargon to help you hack your way through this oily jungle

What is engine oil? In this infographic you'll learn about the different types and what they're made of

Synthetic versus mineral oil

‘Mineral’ oil sounds like it should be synthetic too (minerals aren’t organic, after all), but the name comes from the way it’s extracted from the earth like other mineral deposits. It’s ‘cruder’ than synthetic oil, but also a lot cheaper to manufacturer, and it can still provide perfectly adequate protection for less demanding engines.

‘Synthetic’ motor oil is the pinnacle of engine lubrication for high-performance vehicles. Despite the name, though, synthetic oil is still derived from the thick black stuff ejected by oil wells. The difference is that its molecular structure  and properties are modified, refined and ‘synthesised’ using complex laboratory processes. Synthetic oil can be made more durable, particularly for the ‘shear’ stresses in transmission systems, and allows lower SAE ‘W’ grades for better performance and protection at low temperatures.

‘Part-synthetic’ oils lies somewhere in the middle, offering much of the performance of fully synthetic oils but at a much lower cost.

So it sounds as if mineral oil comes from the ground and synthetic oil comes from a laboratory, but in fact they’re both products of crude oil and it’s just that synthetic oils offer a step-change in sophistication, performance and, alas, expense. Much modern engine technology has only become possible because oils have been developed to support it.

Viscosity ratings explained

For oil to offer the right protection and performance it has to ‘flow’ properly through and around the engine components. The technical term is ‘viscosity’ and it’s traditionally been measured at 100 degrees Centigrade. This gives an standardised SAE (Society of Automotive Engineers) rating so, for example, a relatively thick, old-fashioned motor oil might have a viscosity of SAE 50.

But car engines operate across a range of temperatures, particularly when they start up ‘cold’. An oil designed for normal engine operating temperatures is much too thick when the engine is cold, which has traditionally been when most engine wear happens.

The solution is ‘multigrade’ oils which use VII additives (Viscosity Index Improvers) to make the oil run more freely at low temperatures. To measure this, a second low-temperature ‘winter’ (W) test is added, so you might get SAE 20W 50 multigrade oil, for example.

Basically, the lower the ‘W’ number, the better the cold performance and the higher the second number, the better the oil copes with high temperatures.

But SAE 20W 50 motor oils are old-school, and engine tech has moved on. Modern engines have much finer tolerances and need much better low-temperature performance. It’s not uncommon for makers now to specify a 5W 30 motor oil even for a regular family car.

Regular mineral oils can only adapt so far. To get to these really low ‘W’ ratings needs the precise chemical formulations of fully synthetic oil.

What else goes into your oil?

It’s not just about oil grades and synthetic versus mineral oil – engine oils are like a complex chemical soup designed not just for lubrication, but protection, engine efficiency and specific adaptations for modern innovations like diesel particulate filters.

So that can of engine oil you just bought almost certainly has some or all of the following blended in: Viscosity Index Improvers (we’ve talked about these), detergents, dispersants, anti-wear agents, friction modifiers, pour-point depressants, anti-oxidants, foam inhibitors and corrosion inhibitors.

So which oil is best for your car?

Start by checking your owner’s manual or the car maker’s website. Often, they will quote specific OEM (original equipment manufacturer) standards which are then picked up on by the oil companies. 

Here are just a few: BMW LL-04, Citroen/Peugeot PSA B71 2312, VW 501.01-505.00, Renault RN0700, Ford M2C917-A.

Now it might look from those serial numbers as if there could be thousands of these manufacturer-specific codes, but actually there aren’t. These codes are assigned by technicians who have no idea how much they’re scaring the rest of us, and there really aren’t that many of them. So if you find the maker’s OEM code for your car and then Google it, you’ll probably find that a whole bunch of big-name oil companies quote that directly on the oil carton or in the small print on the back.

There are also independent bodies which specify oil standards. In Europe it’s the ACEA (Association des Constructeurs Européens d’Automobiles) and in the US it’s the API (American Petroleum Institute). These bodies routinely update standards for oil performance as car technology advances, and both offer separate standards for the different needs of petrol and diesel cars. 

In fact, most oils are designed to be used in both petrol and diesel engines, and the makers will quote a combined petrol/diesel code on the carton. In Europe, you might buy an ACEA A3/B4 oil, for example, and in the US you might get an API SN/CJ-4 motor oil. If you can’t find an OEM code for your car, you can often work out the oil you need from these codes

But what if your engine is old?

High-priced synthetic oils are not always best. Older engines may have been designed at a time when tolerances were much wider, or general wear will have made the tolerances wider – either way, a traditional, more viscous mineral oil won’t just be cheaper, it might offer better protection.

Many oil makers produce oils designed specifically designed for older, worn engines, with special ‘seal-swelling’ additives designed to reduce internal and external oil leaks. ‘High mileage’ oils might not just be the cheaper option, they might be the best one.

So there is actually a lot more to engine oil than you might imagine.