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Oil is the lifeblood of an engine. Besides its primary function of providing a lubricating film between moving metal surfaces, it also performs a variety of other essential tasks:
l It keeps the inside of the engine clean by flushing away unwanted deposits and microscopic metal particles. The metal particles are removed by the filter and the deposits and any contaminants which might find their way from the combustion chambers past the rings, are either dissolved or kept in suspension in the oil until the next oil change.
l It prevents corrosion, the ever-present danger when oxygen is around hot metal surfaces.
l It maintains a seal between the piston rings and the cylinder walls to prevent "blow-by".
l It helps to cool the engine by carrying heat away from the hottest parts, to an oil cooler or oil pan and filter where the oil can be cooled by airflow.
Ordinary mineral oil can't cope with these demands. Oil has a natural tendency to thicken as it gets colder and to thin out as it gets hotter. This causes a dilemma for an engine. Oil that's thin enough to flow easily at start-up on an icy winter's morning will become so thin when the engine reaches operating temperature that the lubricating film will break down. Oil that has enough viscosity at operating temperature will get so thick on a freezing morning that the oil pump would hardly be able to move it throughout the engine.
To counteract the problem, manufacturers add special polymers to the base oil to stabilise the oil's viscosity and make it a multigrade oil. So a 10W-40 oil will be as thin as a 10 grade oil at low temperature, but as thick as 40 grade oil at 100EC. It will still become thinner as the temperature rises, but the change will be smaller than without the stabilisers.
A good quality oil will have a carefully balanced additive package, containing not only viscosity stabilisers, but also oxidation and corrosion inhibitors, detergents and dispersants and high-pressure agents.
Much of the oil-maker's skill and research go into putting together this additive package, which typically makes up about 20percent of the overall volume of the final product.
Oil companies take a dim view of miracle additives sold over the counter which might upset the delicate balance that they have achieved with all their hard work.
The oil scene, always a fertile breeding ground for fierce controversy, fuelled by way-out theories, unsubstantiated rumours and quasi-scientific personal observations, became even more complicated in the early 1970s with the arrival of the first "synthetic" oils.
Since then they have steadily increased their market share. They consist of chemical compounds which are not present in crude oil, but artificially made in a factory. To further confound the issue, certain oil companies subsequently developed processes whereby refined crude oil can be transformed into high-quality mineral lubricating oil which performs like earlier synthetic oils, but are cheaper to manufacture.
Synthetic oil has advantages over mineral oil, but also drawbacks. We shall look at these next week when we give guidelines for choosing the right oil and consider how often engine oil and oil filters should be changed.