See Figure 1
The SAE (Society of Automotive Engineers) grade number indicates the viscosity of the engine oil and thus its ability to lubricate at a given temperature. The lower the SAE grade number, the lighter the oil; the lower the viscosity, the easier it is to crank the engine in cold weather.
Oil viscosities should be chosen from those oils recommended for the lowest anticipated temperature during the oil change interval.
Multi-viscosity oils (10W-30, 20W-50, etc.) offer the important advantage of being adaptable to temperature extremes. They allow easy starting at low temperatures, yet they give good protection at high speeds and engine temperatures. This is a decided advantage in changeable climates or in long distance touring.
The API (American Petroleum Institute) designation indicates the classification of engine oil used under certain given operating conditions. Only oils designated for use "Service SF" or "Service SG" should be used. Never use oils rated lower, though it is acceptable to use oils rated for use in diesel engines as long as the oil has a SF or SG gasoline rating also. Oils of the SF/SG type perform a variety of functions inside the engine in addition to their basic function as a lubricant. Through a balanced system of metallic detergents and polymeric dispersants, the oil prevents the formation of high and low temperature deposits and also keeps sludge and particles of dirt in suspension. Acids, particularly sulfuric acid, as well as other by-products of combustion, are neutralized. Both the SAE grade number and the API designation can be found on top of the oil can.
For recommended oil viscosities, refer to the chart.
Do not use engine oil additives, as today's oils perform all the tasks necessary to maintain the internal condition of the engine. Modern synthetic oils can provide additional engine protection due to the superior lubricating properties of these oils. Low friction synthetic oils are rated in the same way as standard oils, so all the guidelines, service and viscosity ratings used for standard oils apply to synthetics. Use low friction synthetic oils approved by BMW. Your dealer can provide you with an up to date list of approved synthetic low friction oils.