The Synthetic Advantage Why is synthetic engine oil better than conventional oil? There are two basic reasons. Synthetic engine oils provide improved low temperature pumping and cranking and greatly improved high temperature durability. What this means to the consumer is improved engine starting and protection at very low temperatures and improved high temperature protection against oil breakdown and sludge. Now explaining why this is the case can be rather complicated. Let’s start with understanding the difference between synthetic engine oil design and conventional oil design. Synthetic vs. Conventional Engine Oil Design Synthetic and Conventional oils are formulated in the same fashion. Various chemicals (additives) are added to base oil to provide all the protection that your engine needs. The additive technology used for synthetic and conventional oils is very similar. Dispersants, detergents, oxidation inhibitors, anti-wear additives, etc. are all used for both synthetic and conventional oil. For many synthetic oils, however, the actual additives used in the formulation may be higher quality and the may be used in higher quantity. This is an issue we’ll deal with a bit later in this article. Since the additive technology is basically the same for synthetic and conventional oil, the difference is in the base oil. Synthetic base oils are engineered to be purer substances with much tighter control of the actual molecules used in the oil. Conventional oils on the other hand are extracted from crude oil in a refinery where they are cleaned and processed to make an oil that is the appropriate viscosity for formulating engine oil. Conventional oils have a broad mix of hydrocarbon structures; some are small molecules, some are big. Ultimately, this broad mix of molecules can have a detrimental effect on the long-term performance of conventional oil. The Low Temperature Advantage Why is synthetic engine oil better at low temperatures? Many people over the years have incorrectly assumed that synthetic engine oils must be thinner (lower viscosity) to provide this enhanced performance at very low temperatures. They are dead wrong. Remember from Engine Oils 101 that oils are characterized by their SAE viscosity. A 5W-30 synthetic oil has the same viscosity as a 5W-30 conventional. The reason synthetic oils flow better at low temperatures is because they have no wax molecules in the base oil. Conventional oils on the other hand are basically cleaned up crude oil which has a considerable amount of wax. If you have ever bought paraffin wax or candle wax at the store, it most likely came from the same refinery where conventional base oil was produced. One of the steps to produce conventional oil is to remove the waxy molecules. Unfortunately, it is difficult and expensive to remove every last bit of wax from the oil, so some wax is always present in the finished product. Since wax will form a crystalline structure at very low temperatures, conventional base oil will actually solidify at about 0°F. An additive called pour point depressant will help lower the pour point down to around –30°F in the finished conventional product. However, these waxy molecules can contribute to reduced pumping capability, even at temperatures above the pour point. Since synthetic base oil has no wax molecules, there is no opportunity for wax crystallization. Synthetic engine oils generally have extremely low pour points down to –65°F. This is due to the fact that synthetic base oils are made up of essentially one type of molecule. It is a pure substance designed to have excellent flow characteristics. Good low temperature flow is a huge advantage for engine protection. Most engine wear can occur during the short period when the engine has started and is running but the oil has not come up to operating pressure. Synthetic oils can shorten this period of time and help protect against this type of wear. Additionally, if you live in a very cold climate, where the temperature regularly drops below 0°F, synthetic engine oil can be a vital component to allow engine starting and extending the life of your engine. The High Temperature Advantage Synthetic engine oils also provide optimum protection against engine oil breakdown due to high temperature operation. Synthetic base oil is specifically engineered to resist thermal breakdown. Conventional oil, on the other hand, can only provide satisfactory protection under normal engine operation. The difference again is in the molecular structure of the oil. Conventional oil has a broad molecular weight range of molecules. What this means is it has small, medium and large molecules. In general, the bigger the molecule, the more viscous the fluid. The medium molecules are really the appropriate size and viscosity for making engine oil, while the small and large molecules just cancel each other out with respect to viscosity. There are problems with the small and large molecules, however. In a hot engine, the small molecules will tend to evaporate which causes oil consumption. It also causes viscosity increase because there are less small molecules to cancel out the large molecules. The large molecules will tend to have some weak molecular bonds, which will cause the molecule to break or shear. Once a molecule has sheared, it has an open bond available. Free oxygen can tie up this bond and start a process known as polymerization. This means that the molecule will start to grow as it hooks up with other free molecules. Before you know it, you end up with a really huge molecule, which will cause viscosity increase and eventually become sludge. Synthetic oil is made up of one type of molecule, which has been tailored to have the right viscosity properties and very strong molecular bonds that resist shearing. Since there are no small molecules to evaporate, the oil provides excellent oil consumption control. The strong molecular bonds prevent polymerization due to high temperature oxidation. This helps prevent viscosity increase and sludge. Synthetic Additive Formulation Although synthetic and conventional oils are formulated in the same fashion, there are some subtle differences that can affect the performance of the finished oil. The additives used in synthetic oil are generally of higher quality such that they provide enhanced protection. The additives used generally have better low temperature properties so as not to disturb the low temperature performance of the base oil. The cost structure of a full synthetic oil also allows for the use of higher quality and higher cost additives. When an additive is added to an oil, it displaces some of the base oil. In the case of conventional oil, the higher cost additive is displacing very inexpensive base oil. This drives up the cost of the oil. To be competitive, conventional oil is produced with the absolute minimum additive required to provide minimum protection. It is still a good quality product since the minimum performance is quite high, but it could easily be better if more additive were used. On the other hand, synthetic base oil is expensive. So the addition of more expensive additive is not as cost prohibitive. This allows the oil to be formulated to a higher standard with better additives and more of them. That’s not to say all manufacturers will do it this way. Some manufacturers will still use the conventional additive treatment to save cost. Is Synthetic Oil Right for You? It really depends on your driving habits. If you have a standard commute with moderate ambient temperatures, you could make out fine with conventional oil so long as you change it regularly. However, if you live in a very warm or very cold climate and you tow a load or run your engine hot, you should probably consider synthetic oil. It’s actually inexpensive insurance for your engine in case it ever overheats or is subjected to very cold startups. Which Synthetic Oil Should I Use? This is a tricky question. There really is not much of an industry standard definition when it comes to synthetic oil, so be careful. The term synthetic has become more generic these days and has more to do with oil quality than it does with the actual oil itself. There is a brief description of some of the base oils used for synthetic oil in Appendix 1. You may want to do some research to verify the quality of a brand of synthetic oil. If you want a true high performance synthetic, you probably want an oil which uses PAO base oil. PAO is the classic synthetic fluid used to make synthetic engine oil and it has excellent low and high temperature properties. The easiest way to verify the use of PAO is to ask the manufacturer for the pour point of their oil. If the pour point is –54°C (-65°F) or lower, then you’ve got a PAO based synthetic oil. Also, if your vehicle is still under warranty, be sure that the oil you choose meets the API service category noted in your owners manual. Failure to do so may invalidate your warranty. Appendix: Synthetic Base Oils
Polyalphaolefins (PAO) Synthetic Esters Hydroprocessed Mineral Oil Over the past 25+ years, PAO has been the most popular fluid used for synthetic engine oil. PAO is a pure chemically engineered substance. It starts by chemically reacting pure ethylene (an invisible, explosive gas) into a molecule known as decene (a clear, thin liquid). The decene molecules are then reacted to form a pure molecule known as Polyalphaolefin (PAO). The PAO molecules can be small or large depending on how much decene is reacted. The bigger the PAO molecule, the more viscous it becomes. In the end, you have a pure chemical substance with very strong molecular bonds that resists physical and chemical breakdown. PAO is usually combined with a small amount of synthetic ester to provide even greater high temperature durability and improved chemical solubility. Synthetic esters are manufactured by reacting an acid and an alcohol. The reaction products are water and ester. Esters are very stable at extremely high temperatures and are used almost exclusively in jet engines. Unfortunately, full ester engine oils do not work well in automobile engines due to lower operating temperatures and exposure to moisture. If ester is exposed to high moisture at moderate temperatures, it tends to react back into the acid and alcohol. You can imagine what acid in your crankcase would do to your engine. Hydroprocessed Mineral Oil is a more recent fluid used for synthetic engine oil. This fluid is actually just conventional oil, which has been processed to remove more of the small and large molecules. In the end, the fluid is much more stable at high temperatures and provides good service for high temperature engine operation. Unfortunately, it provides little or no improvement for low temperature operation since it still contains wax. Since this fluid provides good performance at a lower cost than PAO/Ester formulations, it has found its way into the marketplace as synthetic oil. It is somewhat questionable how synthetic this fluid is.
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