WHAT ENGINE OIL DOES
Without a suitable lubricant, an internal combustion engine would not last for long. A complete loss of oil pressure results in the heavilyloaded metallic components to contact, overheat and seize. Many parts of an engine have different lubrication demands than others and the main practical issues affect classic car owners, where certain anti-wear additives that protect older engines are either reduced, or removed, for environmental reasons. However, this does not always mean you should pay a premium for oils with these additives put back in: ZDDP (Zinc Dialkyldithiophophate) is a typical example.
Aside from preventing metal-to-metal contact, the oil must also transfer and shed heat effectively. However, this and lubrication are not its only roles. Certain components rely on the pressurised oil to provide an hydraulic function, such as valve lifters, variable timing variators and even timing chain tensioners. Some of these components can boast small filters that might block, especially if the car’s service history is patchy.
As petrol and diesel are a complex mixture of compounds, they do not burn entirely cleanly. The engine oil must remove these deposits and keep them in suspension, so that they do not build within the crankcase. The deposits are one reason why the attractive golden hue of fresh oil turns black as it ages, especially on diesel engines. However, should the engine have a fault, or the oil has not been changed on time, the lubricant reaches a saturation point; the deposits can then fall-out of the oil and can start to build within the engine. The most common example is a sticky, plastic-type sludge that can not only block the oil galleries and affect the valve lifters (causing a loud rattling from the top of the engine) but it also compromises the oil’s protective qualities.
Besides, modern engines also require chemical protection. While burning a litre of petrol produces, approximately, a litre of water that promotes rusting, addressing acidity is a further challenge. Ethanol-laced fuels, for example, lower the ph value of the oil, which is why the latest oil blends include extra additives that can resist petrol containing up to 20% Ethanol. Oils also keep rubber seals supple, reducing hardening and, therefore, the opportunity for leaks to develop. The lubricant must also maximise the life of the anti-pollution systems, including catalysers and DPFS.