The engine oils on the market today come in a wide range of manufacturers and properties. This quickly leads to the question – which oil would be best for my car? The simple and only correct answer to this question is – the one recommended by the vehicle manufacturer in terms of characteristics and gradation.

You can easily find which oil is right for your vehicle in any owner's manual online. Depending on the conditions of use, manufacturers recommend only two or three types of oil, depending on the climate and how the vehicle is used as well as the oil changing interval. These specifications are given in accordance with SAE, API or ACEA terms and conditions and imply that the oil has the properties prescribed by those organisations. ILSAC or JASO designations are also sometimes found. These are the acronyms of an association – an association of automotive engineers and companies that test and standardise all manufacturers' oils to meet the standards set by engine design and environmental requirements. SAE and API are American associations, ACEA is European, ILSAC is American-Japanese while JASO is Japanese.

Oil packaging bears the labels and standards directly prescribed by the vehicle manufacturers, so in addition to the basic specification, we often find labels such as VW, BMW, etc. In this case, it is not a specification, but an OEM (Original Equipment Manufacturer) approval. It is important to note that VW approval is not valid for use with other manufacturers. With such meaningful but clear labelling, everyone can find the oil that is right for their vehicle's engine.

SAE "5W30" is the most commonly used type of oil in our country, while other standards are usually indicated on a sticker on the back or in smaller letters below the SAE designation. Labels such as "5W30" indicate the viscosity of the oil at low and high temperatures. You can easily find out the temperatures in the tables online. As a rule of thumb: as the number to the left of the letter W is lower, you can apply the oil at a lower temperature. However, the “0” mark does not mean “zero” degrees, it is an oil suitable for engine operation down to -40°C, while the 5W mark is suitable down to -35°C. The same applies to the number to the right of the "W". The higher the number, the higher the outside temperature at which the oil can be used. Simply put, the label on the left gives the viscosity value at about -18°C, the right label at about 100 °C. The lower the number, the thinner the oil and therefore the easier it flows, but even at high temperatures it still maintains a sufficient lubricating film.

Oils have the difficult task of retaining their lubricating properties under thermal loads, but also when the engine is running cold, allowing an oil film to form between the parts that touch. This is also very important for engine cooling. The designations of other standards effectively say the same as SAE designations, so they are their equivalents in another measurement system. API uses "S" followed by the letter below, or "C” with an additional letter. These are quality and type designations, where "S" stands for petrol engines and "C" for diesel cars. The "S" ranges from "A" to the higher "L" quality, indicating higher quality and lubricity. Diesel oils with a "C" rating are graded from basic "A" to higher quality of "G"'. Similarly, other standards are almost always printed side by side on the packaging, so you can use this to check if the oil you have chosen is the right one. For example, with the diesel particulate filters introduced and the increase in the change interval, VW introduced the 504-507 standard and any oil with this designation, regardless of manufacturer, has been reliably approved by VW.

Until the mid-1970s, the predominant oils were those whose base was obtained by refining petroleum from natural raw materials – and these are the so-called mineral oils. Basically, each oil consists of a base oil to which various additives have been added to improve properties, remove impurities, provide additional protection against corrosion, improve viscosity, prevent foaming under load, etc. 

But the base oil still determines the basic and main characteristics. To improve viscosity and properties at low and high temperatures, mineral base oils were slowly replaced by semi-synthetic and then fully synthetic oils. In comparison, mineral oils have been labelled up to 15W40, semi-synthetics up to 10W40, and synthetics up to 0W30, but the most common labels are 5W30 or 5W40. The biggest and only disadvantage of synthetic oils is their higher price, but due to their improved molecular formula and the aforementioned improved viscosity, they do not oxidise or damage seals.

And semi-synthetic oils? These are oils made up of synthetic and mineral oils used in engines of more than 200,000km and are older than 10 years. However, these oils are not used because of the age of the vehicle, but because of the specifications that were current at the time the vehicle was manufactured. But be ware: if you mix synthetic and mineral oil yourself, you will not get a semi-synthetic or an oil of comparable quality.

You can even mix oils of the same gradation from different manufacturers, which means you can add 5W30 from one brand to 5W30 from another – if you can't find the same oil. However, never mix oils of different gradations, even if they are from the same manufacturer, as there is a risk of a chemical reaction. Due to the different "additive cocktails" in different oils, an oil of a certain gradation may lose its properties when mixed with an oil of a different gradation.