Car Care

Knowing your engine oil


While most people know to check the owner’s manual to find out which kind of oil their car takes, we still get a fair few questions from AA Members that show there is still some confusion about the different oil grades, codes and qualities.

Numerical codes

Single and multi-grade oils include numerical codes in their designation which represent the oil’s viscosity grade or, in other words, the likelihood of the oil to flow quickly or slowly. The lower the number, the thinner the oil will be. On multi-grade solutions, the second figure reflects the performance of the oil when hot.

Take 10W-30 for example. The ‘W’ in this common oil designation stands for ‘winter’. The 10 indicates that the oil can still be pumped by the engine as low as a single grade 10W oil, and the lower the number, the better the oil’s cold temperature or cold start performance will be. The 30 represents the higher temperature viscosity and indicates how well the oil will flow when heated to 100 degrees Celsius. 10W-30 therefore offers the best of both summer and winter viscosity characteristics, eliminating the need to use separate oils, as was done in the past.

Synthetic oil

As well as knowing your petrol and diesel-suited oils, it pays to be aware of whether your vehicle does or doesn’t require synthetic oil.

All oils are made from crude oil but the differences between mineral and synthetic oils are defined by the manner in which they’re refined and the additives they include.

Synthetic oils however are derived from a more advanced refining process which removes more impurities from the oil and also enables individual molecules in the oil to be tailored to the demands of the modern engine. This purer oil is generally of a higher quality, and thus offers higher levels of protection due to increased anti-friction properties.

The bottom line is that it can translate into less engine wear, improve your fuel economy, and prolong engine life.

It goes without saying that the higher quality of oil you choose, the more expensive it’ll be. If your vehicle is older, has a significant number of kilometres on the clock or has an engine that burns a lot of oil, you may not want to spend so much on synthetic oil.

Diesel oils

For a number of reasons it is best not to use diesel oil in a petrol engine, even if the viscosity rating is the same, as the oil properties may not be suitable.

The demands placed on a compression ignition diesel engine mean that a slightly different type of oil is required than that of a spark ignition petrol engine. Diesel engines create a great deal more soot and combustion by-products so their oils have additives like detergents, which neutralise acids and keep surfaces free of deposits in order to keep the engine clean. When put in a petrol engine, the detergent will work as it is designed and try to clean the cylinder walls. Over time this can have an adverse effect on the engine internals, resulting in lost compression and efficiency.

Oil viscosity is also a major factor here. Engine oils need to be designed so the oil will easily pump around the engine at the lowest start-up temperatures while still protecting the components at in-service operating temperatures. Traditionally diesel oils can have a higher viscosity, and this could play havoc during the cold starts as the oil would become thick and not flow very well, subsequently resulting in premature wear.  

Diesel oils also contain high anti-wear additives (zinc dialkyldithiophosphate or ZDDP) as a small amount of oil is burned during the emission process. Diesel catalytic converters are designed to cope with this combustion by-product and petrol systems are not.    

While brands and prices vary, it’s always best practice to find out what oil the vehicle manufacturer recommended when the car was built and use this, as it’ll be based upon the properties the oil offers that are best for the engine.

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