All about engine oil

It’s a slippery subject, but here’s what you need to know to keep your engine lubricated correctly





AA members often ask us questions about oil. It’s key to engine maintenance, but some of the terminology can be confusing. Aside from the obvious petrol vs diesel engine oils and multi-grade uses, there are two main oil qualities based on what they are made up of: mineral and synthetic. Let’s take a look at what the multi-grade numbers and letters on oil packs mean. Viscosity index The viscosity index of oil is rated by its resistance to flow. So, let’s use 10w-30 as an example. This has two viscosity grades, 10w and 30, which gives the best of both summer and winter viscosity characteristics, and eliminates the need for different “winter” and “summer” oils as used in past. In fact the “w” stands for winter, and the lower the number beside it, the better the cold start performance. The second number is the higher temperature viscosity, taken at a temperature of 100 degrees. The viscosity limit is set and all oils (regardless of brand) with a viscosity number must achieve these targets. Once again, the lower the number, the thinner the oil — a 30 weight oil is thinner than a 40 weight oil at 100 degrees, and so on. This is important, as engine oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity oils flow easier to protect engine internal parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures. Mineral or synthetic? Engine oils have different qualities in their make-up and you can get a few grades, just like maple syrup. You can tell the genuine Canadian syrup — it flows nicely over your crepe stack, and the smell and longlasting taste is amazing! You pay more for this luxury, but it’s worth every dollar. On the flipside, you can get a less expensive imitation or maple “flavoured” syrup, which is not quite the same but still has its uses — maybe a child’s party dessert, but certainly not a formal dinner party. All oils are made from crude that comes from the ground. The difference is in the refining and additives. Synthetic oils are made from a more advanced refining process, which removes more impurities from the oil and also enables individual molecules to be tailored to the demands of the modern engine. This 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, better fuel economy and longer engine life. Mineral oils have their place, maybe once a vehicle gets older and the mileage is higher, or if the engine is a bit tired and burns too much oil to warrant using anything too expensive. Half and half Semi synthetic or part synthetic oils are a blend of mineral and synthetic. They provide better performance, protection and fuel economy than mineral oils, but are not as good as a full synthetic. They are a good inbetween solution for those wellkept low mileage cars that don’t quite require full synthetic engine oil. Diesel oils For a number of reasons it’s best not to use diesel oil in a petrol engine; even if the viscosity rating is the same, 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 to a spark-ignition petrol engine, because diesels create more soot and combustion by-product. Their oils have detergents for example, which neutralise acids and keep surfaces free of deposits 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. 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 could play havoc in the cold starts, as the oil becomes thick and can lead to premature wear. Diesel oils also contain high antiwear additives in the form of zinc dialkyldithiophosphate (ZDDP). As a small amount of oil is burned in the process, diesel catalytic converters are designed to cope with this combustion by-product, but petrol systems are not. It’s best practice to find out what oil the vehicle manufacturer recommended when the car was built and use this.