Car Mechanics (UK)
How oil is graded, according to its viscosity (flow). The first number and the letter W represent the grading of oil for winter conditions. The lower the first number, the thinner the oil is, which can be better when the outside temperature drops. An oil beginning with 0W is tested down to –35°C. For a 10W oil, just add 10 onto –35: thus it is –25°C. The second number relates to how thin the oil is at 100°C. The lower the number, the thinner it is at normal operating temperatures.
Association des Constructeurs Européens d’automobiles – the European organisation that classifies engine oil and determines its suitability. See www.oilspecifications.org/acea.php for more information.
American Petroleum Institute – the US organisation that classifies engine oil and determines its suitability.
Semi- and fully-synthetic oils are used in modern engines, replacing mineral oils that are used in older engines. Synthetic oils contain chemicals to help with lubrication, which protects the engine over a far wider range of temperatures compared to a mineral oil. They also have less impurities, so they don’t degrade as quickly, and they’re thinner. Semisynthetic oil contains some mineral oil, whereas fully-synthetic does not. Consequently, fully-synthetic is more expensive.