Purpose of engine oil
In operation, engine oil is exposed to the most uniform influences. Therefore, it is very difficult to accurately determine the effect of various operating influences on engine lubrication. Engines running for long periods of time at high speeds or at full load heat up to high temperatures. Under the influence of high temperature and oxygen contained in the air, the process of oil oxidation begins. Oxidation products thicken the oil and can form varnish-like deposits on the upper parts of the pistons, in the grooves for the piston rings and on the valve stems. This can lead to sticking of the valve heads.
When an over-rich fuel mixture enters the cylinders, or when the engine is rarely or never required to develop full power, or when the engine is often operated in a cold state (short city trips), resulting in incomplete combustion of the fuel. Carbon deposits, oil hydrocarbons, unburned fuel and other products of combustion, as well as moisture condensates cause the formation of sludge, acid and asphalt. Unsaturated fuel gets on the cold walls of the cylinders and flows down them into the crankcase, while the oil film is washed off the walls of the cylinders and pistons. The consequence of this is the deterioration of the lubrication of the cylinder walls and the dilution of the oil, which affects its lubricating properties, depending on the amount of fuel that has entered it.
In case of severe dilution of the oil, it should be replaced ahead of schedule. Since with a sharp driving style (hot engine) gasoline particles evaporate in the oil, the oil level should be checked more often in winter (numerous cold starts - high proportion of gasoline in oil).
Engine oil viscosity
The viscosity of engine oil changes with temperature. As the oil heats up, it thins out. This affects the adhesion and pressure resistance of the oil film. As the oil cools, the viscosity increases, and the internal frictional resistance increases. This property requires the use of an oil with a viscosity that changes as little as possible with temperature changes.
On a cold engine, the oil should be thin enough. in order not to impede the operation of the starter, penetrate as quickly as possible to all lubrication points.
Viscosity is identical to the internal friction of the oil and is classified according to the SAE system (Society of Automotive Engineers), such as SAE 30, SAE 10, etc. High indexes are assigned to thick oils, low indexes to liquid ones.
Universal (all season) oil
All-weather oil must be used for BMW engines. Multigrade oils have the advantage that they operate over a wide temperature range (summer Winter). The basis of all-weather oil is a grade of liquid oil (e.g. 15W). Thanks to the so-called hot thickener, the oil is stabilized, thus retaining its lubricating properties under all operating conditions. When using multigrade oils, modern grades with a wide temperature range of viscosity should be chosen (For example. 15 W-40, 15 W-50).
Letter «W» in the SAE designation indicates the suitability of the oil grade in winter.
Light oils
With low-speed oils, we are talking about all-weather oil with an additive that reduces internal friction, which can save up to 2% of beer. Lightweight oils have a low viscosity (e.g. 10 W-30). Unusual oils are used as base oils here (synthetic). When buying low-speed oil, you should pay attention to the BMW approval (written on the label).
Application ranges/Viscosity classes
Since the ranges of application of seasonal oils of neighboring SAE classes overlap, short-term temperature changes can be ignored. It is allowed to mix oils of different viscosity grades if topping up of oil is required, and the ambient temperature no longer corresponds to the viscosity grade of the oil filled into the engine.
Additives - of any kind - must not be mixed into either the oil or the fuel.
Engine oil specification
For modern engines, only HO oils are allowed. HD oils are alloyed oils whose lubricating properties are greatly improved by the addition of various chemicals. These additives provide good anti-corrosion protection, reduction of oxidation activity, especially low tendency to form sludge in the crankcase, better viscosity characteristics, cleaning and dissolving properties. Cleaning and solvent additives not only reduce the formation of deposits in the engine, but also have the ability to dissolve deposits and permanently keep them. as well as other contaminants in the engine oil in a state of suspension, so that when changing the oil they drain along with the old oil.
Parameters of HO oils are classified according to the API system (American Petroleum Institut). European manufacturers also follow this system.
Designations are two-letter. The first letter characterizes the range of application: S - Service, designed for gasoline engines; C - Commercial, designed for diesel engines. The second letter in alphabetical order characterizes the type of oil.
The highest grades of oils are SG oils for gasoline engines and CE oils for diesel engines (according to the API specification).
Attention: Engine oils clearly marked by the manufacturer as oils for diesel engines are not suitable for gasoline engines. There are oils that are suitable for both gasoline and diesel engines. In this case, both specifications are indicated on the packaging (For example. SF/CD).
The following oil grades are suitable for BMW vehicles with petrol or diesel engines: SF/SG, SG/CD or SG/CE API specification oils.
In any case, you need to pay attention to the fact that the purchased oil is approved for use on BMW cars (written on the packaging).
Oil consumption
In internal combustion engines, oil consumption refers to the amount of oil that is consumed due to the combustion process. Oil consumption cannot be equated with oil leaks occurring in the oil pan, cylinder head cover, etc.
Normal oil consumption is obtained as a result of the combustion of a small amount of it in the cylinders and its oil along with the objects of combustion and friction. In addition, oil consumption occurs due to high temperatures and pressures constantly affecting the engine.
In addition, external operating conditions, driving style, as well as manufacturing tolerances affect oil consumption. Oil consumption should not exceed 1.5 liters per 1000 km.
When the oil drops to the level «Top up» be sure to add oil (the amount of oil to be added is max. 1 liter).