The slippery liquid stuff that we pour expensively into our engines at every service serves very many purposes, some of them not so obvious. Lubrication is the obvious and most important purpose. That aside, engine oil also acts a cleaning agent, washing away the filings and worn metal particles that come with two surfaces rubbing against each other without lubrication. The oil film over the metal surface also inhibits rusting by preventing oxygen from getting at the metal, a property supported by anti-corrosion additives added to the oil. The same oil also acts as a heat sink by convection, dumping the friction-induced heat into the oil cooler or oil sump. The oil sump is the finned pan typically seen hanging on to the bottom of an engine, and acts as storage for the oil when it is not circulating. Heat is lost from the oil in another way through the PCV system (Positive Crankcase Ventilation) when oil gases built up in the crankcase are evacuated, carrying heat away with them. Simple.
Where and how is the oil is applied, however, complicates things slightly, and this is where we dive in head-first to demystify the technical stuff.

First off: how to get the oil:

Oil Manufacture
The oil used in the engine typically has its base extracted from crude oil through the refining process. Since this is the same crude from which we get everything from creosote (tar) to kerosene and jet fuel, extracting that oil base has to be done by fractional distillation. This is a process similar to simple distillation in that it involves boiling the living daylights out of the black gold, but there is a small addition, the addition of a veritable obstacle course that goes by the blood-curdling name of a fractionating column. That fractionating column provides a large surface area for cooling at minute temperature differentials, allowing the tapping of different materials with different boiling points. At the base of the column, the heavy oils and greases are extracted, somewhere just above that is where the oil base stops its journey. Higher up is the diesel tap, among others, while the light, volatile liquids make their way through the maze and are tapped closer to the very top.

To this extracted oil base are added a number of chemicals, what we call oil additives. They include corrosion inhibitors, anti-coagulants, dispersants, detergents and a lot of other stuff that give the oil its cleaning abilities, increase the operating temperature range (by lowering the freezing point and raising the boiling point), changes the viscosity index and prevents the oil from forming sludge in the sump. The resultant soup is what we call mineral-based or regular motor/ engine oil.
However, crude has been getting increasingly expensive; and its availability subject to either the whims of a clique of sheiks or the outcome of a needless war, so this prompted some pioneering geeks at Mobil to don their white coats and work extra hard in the lab finding an alternative base for the oil back in the ‘70s. The result? Synthetic oil, an alternative for the regular, fossil-based stuff. The synthetic oil is made either from select petroleum products (as opposed to whole crude) or other raw materials in a process called hydrocracking that I cannot even begin describing here. It is Organic Chemistry at its most difficult.

The appearance of synthetic motor oil not only provided a viable alternative to motorists, it also formed the base for one of the hottest arguments in the motoring world: which is better, natural or lab-born?

Natural (Mineral) vs. Synthetic
Let me kill the suspense: synthetic oils have the upper hand. They have better performance at extreme temperatures (high viscosity index), they provide longer engine life, their extended change intervals make them friendlier to the environment seeing as they are disposed of less frequently, they lubricate better on cold starts, their physical and chemical properties are superior: they are harder to break down or form sludge, there is less evaporative loss, resist oxidation and have better shear ability. Since synthetics are made in a lab, they have no naturally occurring contaminants, their molecules are smaller, thus making them ideal for today’s engines which have tighter clearances, and they are free of impurities. The thinner consistency of synthetics also provides less drag for the engine to overcome when running.
On the other hand, they are inapplicable to rotary engines. There has been a slow influx of Mazda RX-8 cars locally, which use a Wankel engine (rotary), so if you have bought one steer clear of synthetics. In older models of cars which had to be run in when new, synthetic was not ideal due to its superior lubrication: the desired initial wear on metal parts during bedding in could not occur. Their better chemical abilities also make them prone to decomposition in industrial applications where exposure to other chemicals is a daily occurrence. They also cannot hold lead particles in suspension form (as opposed to solution); meaning cars running on leaded fuel are not ideal candidates for their use. The lightness of the oil makes for lower oil pressure, an unnerving characteristic for petrolheads, but the low pressure has been proved not to have adverse effect on engine life or performance. Their worst trait is the cost: they sometimes cost up to three times as much as regular oil, but seeing that the oil is changed less often, you are the judge as to whether the extra cost is worth it. An example of synthetic oil is the high performance Mobil 1, or Shell Helix.

Apparently, high-mileage older car models are not suited for synthetic oil use as there is a fear that they will burn the oil too quickly. This is because of something called seal-swell property. This is the property of limp seals swelling and getting turgid when washed over with oil. Old cars had bigger tolerances, and running them on earlier synthetics which had lower seal-swell rates caused oil to leak from crankcase and rocker cover seals. At higher seal-swell rates, the seals swelled nicely and fit snugly in place. Running those cars on high-rate synthetic oil was not that bad, but the moment a little mineral oil was mixed in…disaster.

All in all, the difference is negligible as to which is actually better. To get the best of both worlds, there have been semi-synthetic blends, in which mineral oil is mixed with no more than 30% of synthetic oil.

Changing over from mineral to synthetic has caused many an uninformed motorist untold worry. Stop worrying: you can switch back and forth between the two as you please. No flushing is required, nor is it true that once you go artificial you cannot turn back. Some say it is undesirable to dilute the superior synthetic by adding simple regular oil, but at the end of the day, the call is yours.
Other Additives

The other additives have names that sound suspiciously like guesswork. The first is Zinc dialkyldithiophosphate (ZDDP), for protection during extremely stressful operations. It also prevents decomposition by oxidation sludge formation. Molybdenum disulphide reduces friction, yes, but it has the added advantage of bonding to metal. Zinc dialkyldithiophosphate reduces wear between touching metal surfaces by coating those surfaces with a touch of zinc.

Properties of Oil
The first is called the Viscosity Index. Viscosity, as I mentioned elsewhere, is the resistance to flow offered by a liquid, or the “solidity” of a fluid. From elementary chemistry, we know different fluids behave differently when subjected to varying heat levels. Some get less viscous with increase in heat and flow easier, like cooking oil, while others get thicker and more viscous at higher temperatures; like the silicon-based goo that is found in viscous coupling torque-split differentials. In that vein, the Viscosity Index of a fluid is a measure of how the viscosity of the fluid changes with temperature fluctuations: a low viscosity index fluid has its viscosity fluctuating wildly at different heat levels while high viscosity index fluids show little change in the viscosity no matter the ambient temperature.

Some of the other properties include the pour point, which is an index of the lowest temperature of the oil’s utility. The coldest expected operating temperature of the oil is what determines its pour point, since motor oil should still be able to flow adequately during cold starts.
Similar to the pour point is the flash point, the lowest temperature at which the oil gives off vapors which can ignite. The car engine is a bloody hot environment, so a high flash point is sought. This explains the oil extraction process described above, a process which gets rid of volatile flammable oil components and increasing the oil’s flash point.
Oil must also be able to neutralize acids, which sometimes form, and hence have something called the TBN: Total Base Number. The TBN is a measure of the reserve alkalinity of oil: in other words, the oil’s ability to neutralize acid. Similarly, the Total Acid Number (TAN) determines oil’s acidity.

Something called the NOACK Volatility Test determines the degree of physical evaporation loss of oil: after all, oil is a liquid, and it operates at high temperature environments, and we all know liquids evaporate when heated thoroughly. The highest allowable level is 15% evaporation loss, but some manufacturers go for a strict 10% for their cars.
It is these properties that give oil its various designations, like SAE J300, and to explain it away, allow me to introduce you to the Society of Automotive Engineers, or SAE in short.

SAE Jargon
The SAE came up with its own code for quickly determining any oil’s viscosity index. Their grading system starts from low viscosity all the way up to high, with these designations: 0, 5, 10, 15, 20, 25, 30, 40, 50, or 60. From 0 to 25, the rating is suffixed with the letter W, denoting Winter, or suitable for winter use. So an SAE 5 W means an oil with a viscosity index of 5 and ideal for winter. The 20 may or may not have a W, depending on hot or cold viscosity grade.

Multi-grade oils have denotations like SAE 10W-30, which means that it has the viscosity of one grade of oil when cold (10W) and the viscosity of an SAE 30 when hot. This multi-grade oil has the advantage of being useable all year round in extreme climates without having to change. Multi-grade oils can be made by adding VIIs (Viscosity Index Improvers) to regular oil.

While the SAE provides standards for a lubricant’s viscosity, other institutions further grade the oils according to their other properties. The American Petroleum Institute (API) uses the oil’s performance, physical and chemical properties as a yardstick. ILSAC (International Lubricant Standardization and Approval Committee) uses the SAE rating to create their own subsets of grades; grades which are determined by the operating temperature of a given engine. Japanese Automotive Standards Organization (JASO) grades the oils according to whether they are for dry clutch or wet clutch use. To cap it all off, most manufacturers came up with their own criteria to see which type of oil is most appropriate for their motors. They are too many to list here.

Of all these, the SAE rating is the most widely used, and the most relevant to our market.

The 5,000km myth

There has been a furor over whether it is really necessary to change the oil every 5,000km or three months. It has been a common practice for most auto shops to advise clients to do so, but the manufacturers beg to differ, and they are strongly supported by the EPA. The EPA claims that a better indicator of oil change intervals would be considering the following factors: the operating conditions of the engine in question, the type of oil used (mineral, synthetic or blended) and the hours of engine operation. Earlier I had said Daimler, who make the Mercedes C-Class, facelifted their first generation C-Class model with various additional features, one of which was an oil change indicator that rendered the 5,000km service redundant, since the engine oil could now be changed according to necessity and not a timetable. Mercedes claimed this could increase service intervals by up to 80%.

While this makes sense, not all cars have the oil change indicator; and it might be tiring to check your oil physically on a daily basis. A good trade-off would be to make an educated guess based on the three points the EPA raised. So, for short city runs, 5,000km is not bad. For extended light highway use, even 10,000km would be a penny-wise risk. For heavy applications or regular bursts of Rhino Charge-like driving (where water seeps in a lot), maybe sooner: 4000km or so. These figures are here for the purposes of comparison: I have not actually asked anyone to drive 10,000km without changing their engine oil.

source:autobazaar.co.uk