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Choosing The Correct Engine Oil

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Choosing The Correct Engine Oil

There was a time when almost any type or formulation of engine oil could be used in almost any engine, but that time is long past. Today, modern engines demand that oil be formulated in very specific ways to provide adequate and effective lubrication. Moreover, some oils can damage catalytic converters, so if you have ever wondered about what the correct type of oil is to use in your engine, read on and we will tell you all you need to know.


In the world of engine oil, this is probably the most misunderstood term. In simple terms, the word “viscosity” refers to oil’s resistance to flow, and this characteristic is always referred to with the letter “W”, which refers to an oil’s winter, or cold weather flow properties.

Since oil thickens in cold weather, and thus does not flow readily, it must be formulated in such a way that it will not thicken beyond a certain viscosity, or thickness. In standard laboratory tests, oil must flow through an orifice in a test apparatus at specified rates at specified temperatures, and it this flow rate at low temperatures that qualifies an oil as suitable for use in cold weather.

Thus, the “W” is always followed by a second number that indicates how well an oil flows at O0F (-17.80C), which in practice, means that an oil that is labelled 5W-30, will thicken less than another oil that is labelled 15W-30. In turn, this means that the 5W oil is more suitable for cold weather applications because it will remain pump-able at freezing temperatures, whereas the 15W oil will not be.

The number following the “W” rating refers to an oil’s resistance to thinning at a standard temperature of 2120F (1000C), which means that 5W-30 oil will thin out sooner (and more) than 5W-40 oil. In practical terms this means that if you live an area with freezing winter temperatures and scorching summers, you need to use oil that does not thicken too much, or thins out too much at elevated temperatures, and this scenario, you could use 5W-30/40 oil without running into problems.

However, it is always best to stick to the oil rating recommended by the manufacturer of your car, since your car was tested under a wide range of temperatures and driving conditions. It may be that the user manual will recommend a range of suitable oils, so do not use oil formulations that do not fall within this range.

Mineral vs. Synthetic Oil

There are very good reasons why you should use synthetic oil when the manufacturer of your car specifies its use, because the lubrication demands of many high performance engines cannot be met by regular mineral oil.

However, synthetic oil is only synthetic in the sense that it does not contain crude mineral oil. Thus type of oil is distilled in a complex process that uses carbon dioxide, methane, and other hydrocarbon molecules, but the result is oil in which all the molecules are the same size, which means that this oil has almost twice the lubricity of regular mineral oil.

The close engineering tolerances of high performance engines require better lubrication than can be provided by regular oil and using regular oil in an engine that requires synthetic oil will result in the certain destruction of the engine. Moreover, the crank case and emission control systems of these engines are designed to work with synthetic oil only, which means that the catalytic converters of these engines cannot function properly. Mineral oil will clog, or otherwise damage these converters, with possible secondary damage to other emission control related components, such as oxygen sensors.

Nevertheless, the enhanced lubricating properties of synthetic oil make it an excellent choice for use in normal engines that are designed to use mineral oil. The increased lubricity means less friction, which in turns leads to measurable increases in fuel efficiency, lower operating temperatures, and improved throttle response, especially when the engine is cold.

Can I Mix Oils?

It is never a good idea to mix different brands of oil. All oil, whether mineral or synthetic, contains a package of additives that reduce friction, cleans internal parts, inhibits corrosion, and reduces foaming.

The problem with this is that different oil manufacturers do not use the same additives in the same proportions and concentrations. Nor do they all use the same additives, which mean that in almost all cases, the additive packages in different bands of oil are NOT compatible. In some cases, the presence of some friction modifiers can cancel out the cleaning action of detergents in other brands, with a resultant build up of extremely harmful sludge and other contaminants, such as sulfuric acid, several types of varnish, and many different types of wax.
Always stick with one brand, and insist that this brand be used when you have your car serviced at a repair shop.

What to Look Out for In Engine Oil

  • Look for the API (American Petroleum Institute), and SAE (Society of Automotive Engineers) logos or badges on the packaging. All branded oils will have both these logos, and it means that the oil is formulated to perform in accordance with rigorous guidelines and standards.
  • Oil that is labelled with the letters “SL”, conforms to the highest standards of oil and lubrication technology. It contains all the required additives in proportions and concentrations that will provide the best possible protection against mechanical wear. However, do NOT mix brands, because while all “SL” labelled oils will deliver exceptional performance, their additive packages might not be compatible.
  • Synthetic/mineral oil blends are a good choice when the demands on the engine are occasionally increased, such as when towing. This type of oil is more cost effective than fully synthetic oil, but the presence of the synthetic component in the blend adds a significant improvement to the oil blend’s lubricating properties.
  • High mileage oil is recommended for engines with high mileages, and in which there may be significant amount of mechanical wear present. This type of oil does not thin out as much as other oils and they have special additives that “soften” the material oil seals are made from. In some cases, this type of oil can stop oil leaks from seals and gaskets, but this can take some time to happen, and then not in all engines. It is however, a good choice when your engine is old, or has completed a few hundred thousand kms.

What to Avoid

Almost oil manufacturers make oil that can be used in almost all engines, from compact runabouts, to diesel trucks. Dealers often use this type of oil because it is very cost effective for them, but the problem is that it hardly ever complies with the requirements of most engines.

Some engines, such as turbo diesels, require higher levels of detergents and friction modifiers than almost all gasoline engines. High levels of these additives can cause actual harm to some gasoline engines, and while it will not happen overnight, so to speak, the fact is that your engine was designed to use oil in which the various additives must be present in the correct proportions and concentrations.

Using this type of oil over extended periods of time deprives your engine of proper lubrication, and even if you change your oil and filters strictly according to your maintenance schedule, your engine will not last as long as a comparable engine that does not run on all purpose oil.

Look at the label on the packaging, and if it says “Suitable For Use in All Engines”, avoid it like the plague, no matter how much cheaper it is than a properly formulated oil. Rather spend a little more on the oil your engine needs, because if you look after your engine, it will look after you.