What does SAE15W40 mean for My Engine?

By Gwinyai Mataruka

It’s all about viscosity. That’s what they say. They say use thick and strong oil for better protection. Is that so? Why does it seem like the synthetics are very thin?

There are certainly too many fallacies about engine oil. They never end, and will certainly not. The engineers can do their best to formulate oil that will meet different operating requirements, but in the end it’s all a compromise. Their objective is to reduce friction, or lubricate, in order to extend the engine life. The ability to lubricate means that there is always a strong film of oil separating moving metal parts, so that they don’t rub against each other.

However, there are so many environmental and behavioural factors that intervene. Engine oil and the internal combustion engine are a complex interaction that cannot be explained in a one pager. This one will therefore only look at viscosity.

Viscosity simply refers to the thickness of the oil, or resistance to flow, or the time that a drop of oil takes to flow down a vertical tube of a given diameter, from one point to the other. Low viscosity means the oil is thin, whilst high viscosity means the oil is thick. It follows that thin oil (low viscosity) flows faster than thick oil (high viscosity).

The next question to ask is; of what significance is this with reference to engine lubrication? The answer is TIME and SPEED. Research shows that most engine wear occurs during engine start up, especially during what is referred to as “cold starting”, which refers to starting a cold engine. An engine becomes cold when it has been stationary for some time, during which time the oil flows down to the reservoir, or sump, under gravity. When such an engine is started, there is obviously inadequate lubrication, especially for the components at the top of the engine, such as the cam shaft. A desirable characteristic of the oil under cold starting is that the oil takes as little TIME as possible to reach all parts of the engine in adequate volumes and pressure to provide lubrication. A thin oil, or low viscosity oil, will do this much better, and FASTER. It will reach all the engine parts in seconds. This means that low viscosity oil is more effective at reducing wear at cold starting than high viscosity oil.

This is one half of the equation. Before we look at the other half, we need to look at one important characteristic of oil; its behaviour with temperature. Oil has an inverse relationship with temperature. As temperature rises, viscosity falls, i.e. the oil becomes thinner with a rise in temperature. An engine started cold will start rising in temperature when running. The heat is transferred to the oil, which becomes thinner. This affects the ability of the oil to separate surfaces moving against each other, or the ability to lubricate. Thick oil (high viscosity) lubricates better than thin oil (low viscosity) at high temperature. In other words, high viscosity oil tends to offer a stronger oil film than low viscosity oil, everything else being constant. At high temperatures, therefore, high viscosity oil is more effective than low viscosity oil.

Back to our question.

Lubrication engineers simply solved this by creating Multigrade Oil that behaves like low viscosity oil at low temperatures, and high viscosity oil at high temperatures. This is achieved through an additive called a Viscosity Index (VI) Improver. The VI Improver allows a lubricant to have two grades in one oil. At low temperatures, the oil behaves like thin oil, reducing low temperature wear. At high temperature, the oil is thick, reducing high temperature wear.

Having explained thus far, the modern engine oil is generally low in viscosity, but with high film strength. The trend is towards low viscosity oil with high film strength. This is important for fuel economy. Low viscosity oil offers less viscous drag, and therefore serves on energy and fuel. This is achieved by the use of high quality mineral base oils, or synthetics.

The Society of Automotive Engineers (SAE) is the body responsible for automotive oil viscosity grading. For engine oil the low temperature grade is designated by a W, like SAE0W, 5W, 10W, 15W, 20W, and 25W. On the other hand, the high temperature grade is designated thus SAE20, 25, 30, 40, 50, and 60. Multi-grade engine oil will have a combination of any of the two, like SAE0W20, 5W30, 10W40, 15W40, 25W50 etc. Multi-grade SAE15W40 oil will behave like SAE15W viscosity oil at low temperature, or on cold starting; and behave like SAE40 at high temperature.

In general, it means that multigrade engine oil will protect at both low and high temperature, and is therefore a better choice for your engine than monograde oil. However, always check the oil specified for your engine.