(from Safety Fast - April 1992)
Lubricant technology has moved on apace of late, and recently, I realised that I don't really know a lot about some of the terms we bandy about so carelessly. What is the difference, for instance, between a synthetic and semi-synthetic? Should people still be using Castrol oil - good old Castrol R of distant memory? Assuming that I am probably not alone in my ignorance, I recently visited Castrol Burmahs headquarters in Swindon, in order to get the good oil...
There I met Stewart Arklay, Castrol's Technical Manager. He began by explaining what an engine oil is required to do. I was surprised when he enumerated no fewer than five major functions. Obviously, the first requirement of an engine oil is to lubricate, thus preventing metal-to-metal contact. It is also required to protect against rust and other corrosion (from the acids that are an inevitable by-product of combustion); to seal, most obviously at the pistons and rings, but also in such components as the oil pump, for example; to cool (piston crown temperatures can hit 260-280 degrees C); and to clean, trapping both wear debris and the by-products of combustion, and holding them in suspension where they can be trapped by the oil filter or flushed out at the next oil change.
Oils used for lubrication in the internal combustion engine may be divided into three types: mineral, vegetable and synthetic. Some oils are good at meeting one or several of those requirements; not so good at others.
For instance, I mention Castrol "R", a castor oil, which used to be the only choice for a highly-stressed racing engine in years gone by, and which still gives many historic race meetings their distinctive and highly evocative aroma. Castor oil is a vegetable oil, derived from the oil of the castor bean, mixed with some mineral oil. Amazingly, even in these days of highly sophisticated lubricants, castor oil still has some highly prized qualities which are hard to better. It has very high film strength, so it continues to provide a reliable barrier to metal-to-metal contact even under very high load conditions. However, it is rather prone to contamination, and very quickly breaks down to form sludge, lacquers, gums and so on. Hence the need for frequent oil changes.
Despite the availability of more modem lubricants, Castrol R30 and R40 are still in use in some historic racing cars, many of which were of course designed to use castor oil in their heyday. I rather expected that there would be no place for castor oil nowadays: surely modern mineral and synthetic products are much better? Rather to my surprise, Stewart Arklay disagreed: ..If you are happy using castor oil, especially if you have established a maintenance routine of regular oil changes and engine stripdowns and inspections, and it is working well, I wouldn't advise you to change it. So if you are successfully using Castrol ..R. carry on...
However, most of us will be using mineral or synthetic oils. As the motor vehicle has evolved over the years, so too has lubricating oil. Sometimes, the demands of the motor manufacturers have prompted advances in lubrication technology: other times, advances in lubrication technology have allowed the manufacturers to make advances in other areas. In the highest echelons of racing, for example, the recent introduction and successful development of carbon brakes (pioneered in the aviation industry) was made possible only by the development by the oil companies of very advanced wheel bearing lubricants, which could withstand the very much higher temperatures generated.
For the oil companies, the goal posts are constantly moving. Modem engines are built to much closer manufacturing tolerances than of yore. The manufacturers are constantly looking for improvements in fuel consumption and power output and better cold cranking performance and oil flow when cold (did you know that approximately 80% of all engine wear occurs during warm-up?). All of which demand that oils should be thinner. Yet engines generally are designed to run hotter, which suggests the use of a thicker (highly viscosity index) oil. At the same time, they are seeking longer and longer service intervals. Bearing in mind that driving conditions have changed over the years too. Contrast the high average motorway speeds easily attained by the modern motorway when conditions permit, with the agonising crawl of stop start traffic when they don't and the vastly different oil temperatures these two very different driving conditions can generate. All in all, oils have a much harder job to do than ever before. Therefore, oils are constantly changing to keep up with changing demands.
Castrol GTX was introduced in 1968 and became the market leader in 1972; it has been the market leader ever since. But it has changed over the years, to meet changing consumer demands, and the constantly changing specification set by the motor manufacturers and by the likes of the American Petroleum Institute and the Society of Automotive Engineers. For many years it was a 20W50 oil, but is now rated as 15W50. More recently, GTX has been complemented by GTX2, a 10W40 mineral oil to meet the demands of more modern engines; the part-synthetic Castrol TXT; and fully synthetic Syntron X.
Mineral oils fall into two main categories. They are Highly Refined Mineral Oil. which include the likes of Castrol GTX. Duckhams Hypergrade. Shell Super, BP Visco 2000 and Texaco Havoline; and Extra Highly Refined Mineral Oil, including Castrol GTX2, Duckhams QXR, Shell Gemini, BP Visco Nova and Visco 2000 Plus and Texaco Havoline Xl.
All mineral oils have a high wax content which has an adverse affect on the cold cranking and low temperature performance. Solvent refining extracts this paraffin wax, known in the industry as slack wax and the result is known as Highly Refined Mineral Oil. The slack wax is not discarded, however; nothing in this refining process is ever wasted. This slack wax, when treated to a chemical process known as hydrocracking, forms Extra Highly Refined Mineral Oil. Synthetic oils are something else again. Let us clear up one misconception for a start. The very word synthetic is a little bit of a misnomer, in as much as synthetic oils are not the product of a chemist's laboratory: they are all derived from mineral oils. One of the by-products of the petroleum refining process is ethene gas. This gas is transformed by chemical process into what is known as polyalphaolefin synthetic, which forms the basis of synthetic oil. According to Stewart Arklay. the chemical process is almost identical to that used to create certain plastics, so synthetic oils could almost be described as plastic oils. Examples include, of course Castrol Syntron X, Mobil 1 Rally Formula and Fina First. Of all the synthetic oils, Syntron X has the broadest viscosity range, an almost unheard-of 10W60!
There is another category, best described as part-synthetic, the principal examples of which are Castrol TXT and Esso Superlub EX2. These are a mixture of Highly Refined Mineral Oil (like GTX) and polyalphaolefin synthetic, to produce an engine oil with many of the desirable characteristics of a fully synthetic lubricant, but at an economical price. Naturally, all sythetics and part-synthetics, are fully compatible (i.e. they can be mixed) with ordinary mineral oils, if for no other reason than that the motor manufacturer insists on it.
Then, of course, there are the additives. Many years ago, motorists may have used a SAE 40 grade oil in the summer, changing to an SAE 15 product during the winter months. Nowadays, with a multi-grade oil like a 15W40 or a 20W50, you get the best of both worlds. In simplistic terms, it means that a 15W40 oil, when cold, has the cold-cranking characteristics of an SAE 15 oil grade oil, which leads to faster and easier cranking during start-up and quicker oil circulation during initial warm-up: and the characteristics when hot of a SAE40 grade oil.
Every oil, once it is refined to the desired state, then has additives introduced to enhance the qualities it needs to meet the five requirements we first mentioned: to lubricate; protect; cool; seal; and clean.
Detergents are used, which attach themselves to the engine components and prevent anything else from adhering. They are also alkaline, which helps neutralise the acids produced by the process of combustion. Dispersants attach themselves to the tiny particles of dirt and keep them in suspension. Both detergents and dispersants are sacrificial elements, which is why oil, although it doesn't wear out as such, needs to be changed at regular, albeit ever-increasing intervals. Contrary to popular opinion, if an oil turns black and dirty very quickly, that indicates that it is actually doing a good job. Other additives commonly used include anti-oxidants (to inhibit the production of gums and lacquers), viscosity improvers, anti-wear agents and special additives like anti-foaming agents. The so-called additive pack may comprise as much as 12% of the volume of the end product.
Unfortunately, all base oils are not the same: Venezuelan crude, for example, is not the same as North Sea crude. Therefore there cannot be a standard "additive pack". To maintain uniformity of the end product though, Castrol always uses the same source crude for each product.
The above analysis is necessarily brief and simplistic and merely skims the surface of an enormously complex subject. Probably only the oil chemists among us could fully understand it. The rest of us tend to take the contents of our engine's sump for granted. Perhaps the next time you check your dipstick, or treat the car to an oil change, you will appreciate the oil for what it is... possibly the most important component in your engine.