Which engine oil should I use?

As car manufacturers find new ways to increase the performance, lower the emissions and extend the service intervals of their engines, engine oil requirements have become even more complex.

Long gone are the days when a 10W-40 oil was all you needed for almost any car. Finding out exact what engine oil specification your car needs, having a small amount on hand to top it up if necessary and knowing how to check the oil level are all important maintenance requirements for internal combustion engine (ICE) vehicles.

Here we are providing information of a general nature only – you should always check with your vehicle manufacturer or a trusted mechanic if in any doubt about what specific engine oil to use for your vehicle.

Generally speaking, if you fill your engine’s sump with the wrong oil by mistake, it can lead to all sorts of engine problems. These include sludging, seizing, poor performance, higher fuel consumption, oil burning and stalling. Using the wrong oil can also affect the car’s warranty.

If you’ve just topped up your car with the wrong oil, it may not do harm in the short-term, depending on the oil you have used. Engine oils will generally mix together, although the resultant mix may not provide the correct start up or operating temperature viscosity needed for your vehicle, potentially reducing the protection of its internal components. It should be flushed out as soon as possible.

When buying oil, use what the manufacturer recommends. This information can be found in your car’s owner’s manual.

Often manufacturers will provide a few different oil options depending on the climate where the vehicle operates, but generally Australia is considered a hot-weather country (except for regular winter use in alpine regions).

Another way of identifying the right oil is to check with an oil manufacturer; some auto accessory stores also have a vehicle oil guide.

But what if you can’t find this information, or the recommended oil isn’t available for your car? Read on…

Modern engine oil is graded with numbers like 0W-30 or 10W-40, for example, which tell you the high- and low-temperature viscosity rating of the oil, giving an indication of their performance and protection.

The first number refers to the oil’s cold-temperature characteristics (so that’s the 0W or 10W in the examples above). The second number is its viscosity when hot or at the operating temperature of the oil. By the way, the ‘W’ doesn’t stand for ‘Winter’ but the ‘Weight’ of the oil.

A bunch of diligent types called the Society of Automotive Engineers (SAE) beaver away testing oils for their ability to lubricate and flow at a pre-determined temperature. The oils are rated accordingly, using a numbering system from 0 to 60.

The lower the number before the dash (as in ‘0W-’), the more suitable the oil is for lower temperatures. For example, 0W oil will flow at -35°C, 20W oil will flow at -15°C.

The second number (such as the ‘40’ in 10W-40) in a multigrade oil tells you the hot temperature flow rate tested at 100 degrees centigrade. So a 0W-30 grade oil is a 0-weight oil that will not thin out more than a 30-weight oil when it gets hot.

The ACEA (Association des Constructeurs Europeens d’Automobiles) is another bunch of people who, among other things, classifies engine oil types.

So when you see ACEA on the oil bottle label, it will be followed by a specification that starts with a letter, either A, B, C or E. The letters stand for: A – petrol, B – Light Duty Diesel, C – Catalyst Compatible or Catalyst Combustible E – Heavy Duty Diesel.

A common grade, as an example, is ACEA A3/B4 or ACEA C3. Among the sea of numbers and letters, this one is important to ensure you get the right oil. ACEA is the European equivalent of the API (American Petroleum Institute) yet has stricter and more severe requirements on its classifications.

Hence an engine oil that meets both API and ACEA specifications uses a better additive package than one that is designed to meet only API specifications. Some oils may have an ACEA and API specification such as API SN/CF and ACEA A3/B4.

Diesel engine oil has traditionally been designed to cope with the soot generated from the diesel combustion process and exhaust recirculation system, so it contains higher quantities of dispersants and detergents to carry contaminants and prevent oil thickening.

There are also now ‘low SAPS’ oils (also known as low-ash oils), which are engine oils with low amounts of sulphate ash, phosphorus and sulphur. These oils have low-emission additives and are intended for engines with diesel particulate filters (DPF) or three-way catalytic converters (TWC).

When sulphur and phosphorus are burned in large quantities, the residues clog the fine pores of the filters or the lamellas of the catalytic converters. Phosphorus and sulphur neutralise the surfaces of the catalytic converters and prevent exhaust detoxification.

Oil manufacturers for years now have succeeded in developing a range of synthetic oil products with better properties than their natural counterparts – and fewer of their shortcomings.

Very few non-synthetic oils remain on the market and they’re mostly for older cars built before synthetic oils became popular.

There are various reasons why you don’t want to risk using a synthetic oil in an older car, mainly because using it may end up causing engine seals to swell and the engine to leak oil.

Most recent cars have been designed to run synthetic or semi-synthetic oil, which give better performance as they are made using much more advanced chemical processes than traditional engine oil, so they are more predictable in use.

Fully synthetic oils flow more easily at start-up temperatures (which is when most wear occurs). They are less volatile and more heat-resistant than mineral oils. and their antioxidant additives protect against oxidation – a natural process of degradation that occurs in oil.

For example, Shell claims that compared with a normal API SG/CD mineral oil, fully synthetic engine oil delivers up to five times better cleansing, up to three times more engine protection and less than half as much engine wear.