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Take a few minutes to read the Oil FAQ for classic cars.
Whether it’s in the engine or around the gears, the purpose of oil is to reduce the friction between 2 metal surfaces. It does this by sticking to each surface and forming a thin film that keeps the 2 metal components apart. The parts effectively 'float' on a thin film of oil which stops metal to metal contact reduces friction, heat and wear. The quality and performance of an oil is a measure of it’s ability to perform these functions over a long period of time thereby increasing the reliability and performance of the engine components being lubricated.
This is an important question as the correct oil specification will have a significant effect on the performance, reliability and fuel economy of your vehicle. Lubricants are becoming more and more complicated and specific to particular engine types. You need to use the correct viscosity, match the industry standard and follow the manufacturer specifications. Selecting the wrong oil can cause major engine damage and reduce fuel economy, adding to running costs. Using cheap low quality oil is a false economy. Use the Millers Oils WHICH ENGINE OIL guide to find the correct engine oil and gearbox oil for your vehicle.
Viscosity is a measure of the thickness of the oil. The viscosity of oil at low and high temperatures is measured on a scale determined by the Society of Automotive Engineers and given a number known as the SAE number. Therefore SAE 30 oil will be thinner than SAE 40.
Before the 1960s, all oils were monograde. Multigrade oils were developed have a designation in the format 20w50. Mulitgrade oil was made possible by the use of polymers, that expand as they get hotter. Therefore a multigrade oil can have the viscosity of an SAE20 when it is cold and the viscosity of an SAE50 when hot. This removes the necessity to change from thin oil in the winter to thicker oil in the summer, and maintains the best protection for the engine over a wide range of running temperatures. The 20w number is the cold viscosity and the 50 number is the viscosity at 100°C.
Both types are made from crude oil. Mineral oil is obtained directly from the refining process, whereas synthetic oils are altered by chemical processing which improves its chemical structure. Synthetic oils have better performance than mineral oils at all temperatures, and are more durable. They are more expensive but are higher quality, offer higher performance and longer life.
The advantages of synthetic oils are:
• Better flow at low temperatures enabling thinner oils to be manufactured. Thinner oils are required by modern engine designers to help reduce fuel consumption and emissions.
• Higher film strength which gives better protection thereby reducing wear.
• More stable and less volatile at high temperatures, reducing oil consumption.
The interesting 'Race Technology Magazine – Synthetic Oil' article explains the advantages in more detail
As the name suggests, a semi synthetic is a blend of mineral and synthetic base oils. The performance, price, quality and durability is between that of a mineral oil and fully synthetic oil. Lower quality semi synthetic oils have very little fully synthetic content and will not last as well as good quality examples.
Generally the answer is yes for older vehicles, as diesel engine oils have higher detergency levels. It is important to refer to your vehicle manufacturer’s handbook to establish exactly which specification of oil is required. This is vital if your car has extended service intervals and you are still within the warranty period. New vehicles are designed in a way that allows the same oil to be used whether the engine runs on petrol or diesel.
A synthetic oil is produced in a more sophisticated and expensive manufacturing process. A fully synthetic oil does have many performance advantages (see above) which justify the additional cost. Fully synthetic oil allows longer service intervals, reduces oil consumption and contributes to improved fuel consumption – driving running costs down. It is all about quality and performance.