The viscosity has 2 grades: - The cold flow (the numbers before the W of winter): the closer to 0 the number is, the more liquid the oil is at low temperature, which will simplify cold start. - The performance at high temperature: the higher the number is, the more viscous the oil is at high temperature. The information about which viscosity to use for your vehicle is available in your maintenance book.
Classification of the quality of oil delivered by the European Automobile Manufacturers Association (ACEA) There are 3 different groups (A: petrol engine, B: diesel engine, C: vehicles equipped with particle filter and catalytic converter), each one containing several levels of performance (from 1 to 5). Be careful, some oils may not be compatible with your engine, so please refer to the directions written on the maintenance book of your vehicle.