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.
The use of a fuel too low in octane (a measurement of anti-knock quality) will result in spark knock. Since many factors such as altitude, terrain, air temperature and humidity affect operating efficiency, knocking may result even though the recommended fuel is being used. If persistent knocking occurs, it may be necessary to switch to a high grade of fuel. Continuous or heavy knocking may result in engine damage.
The use of 87 AKI (Anti Knock Index) gasoline is acceptable in the 535i, the 1991 525i, the 325iC and the 1991 325i. Use 90 AKI gasoline in all other vehicles. Note, these are minimum ratings and it is acceptable to use higher rated fuels. Always use a brand name, premium fuel.