So, what do you do? Engines have a wide range of start-up temperatures – which even change as the seasons change. The problem is that if you started at 10cSt then as the engine gets hotter (which they do) then your oil would be much too thin when the engine reaches operating temperature. So, the easy way to fix this is to just have the oil be 10cSt at startup right? After all, if the right viscosity for the engine to operate is 10cSt why not just start there and let it be? The engine would be properly lubricated and no harm would result. If it does, the oil might not be able to be pumped into the areas that require it, which can cause more wear, or even cause the engine to break down. At this point, the engine might not even start. This gets increasingly worse as the temperatures drop, and at the low temperature limit for a particular oil, when it gets really cold, it can be over 1,000 times thicker than at operating temperature. If the oil is 10 cSt at 212F, then at 104F (40C) it thickens up to 100 cSt, and at 32F (0C) to 250cSt and so on. ![]() In fact, a basic un-additized mineral oil can thicken so much that it will not lubricate the engine properly at all. In an ideal world the engine oil viscosity would always be 10 cSt, however in real world terms, the oil thickens as the temperature decreases. As an example: If the normal operating temperature hypothetically is 212F (100C) and the engine needs to operate with an oil viscosity of 10 cSt based on the manual recommendation then what does that mean for the engine oil at startup when the engine is not at 212F? What about during the warm up period? What happens to the oil then? Oils recommended given by the engine manufacturer are primarily based on giving the right thickness at the normal operating temperature. If we are interested in engine longevity then we should concentrate our attention at reducing engine wear at startup. It is said that 90 percent of engine wear occurs at startup. It would be more correct to think that oil thickens when it cools to room temperature and THIS is the problem. They think this thinning with heat is the problem with motor oil – but in a properly running motor, engine oil does not get too hot. Sometimes confusion occurs because people think in terms of the oil thinning when it gets hot. But we will talk about that grading system later.įor now, let us just focus on what happens to a basic oil as the temperature changes. This is part of the reason behind why it is so complicated now. Because of these changes the grading system also was changed. Over time various additives were put into engine oils to allow them to do different things, like changing the flow rate as the temperature changed. Around WW-II the Society of Automotive Engineers (SAE) devised a numerical system that would allow a wider range of viscosities to be described based on numbers. Each range of flow rates was assigned a letter A, B, C were different “weights” of oil – or thicknesses. Different oils had different flow rates and there was a very simple way to reference their flow rates relative to each other. In the early days of the automobile, engine oils were very simple, basic refined fluids that helped keep moving parts easily sliding past each other. This is important because your engine – and the oil that is in it runs over a range of temperatures from start up, through the warm up period and on to operating temperature. In engine oil, hot oil has a lower viscosity and flows more easily than cold oil. For most liquids, the flow rate changes as the temperature of the liquid changes. A low viscosity fluid (like water) is thin and flows much more easily. ![]() A high viscosity fluid (like molasses) is thick and does not flow easily. Viscosity is a fluid’s (gas or liquid) resistance to flow. You do not have to worry about what that means, but cSt are different from the SAE viscosity grade. For the purposes of this chapter we will talk about viscosity in the unit it is most often measured centistokes (cSt). In this chapter, we will talk about some basic principles of viscosity and how it relates to your engine. In this chapter, we are not going to talk about viscosity grades – that will be in chapter two. Newer vehicle manufacturers are recommending even lighter grade engine oils, because there are fuel economy advantages to doing so. Others may have a chart which gives recommendations based on the ambient temperatures. For example, they may specify a 0W30 synthetic or a 5W30 mineral oil. Some owner’s manuals allow for different grades based on the predicted operating temperature. The average driver just tries to follow what is in the owner’s manual – this is a good practice. Understanding what viscosity grades mean can be confusing.
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