This info comes from one of the most reputable sources outside of BMW, so I trust it...
http://www.peakeresearch.com/silights.htm
Service Lights Explained
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SI Lights
The Peake website information is true, but outdated. Ron Stygar's pages include this snippet by BMW themselves, detailing the wa in which the later SI calculation are made:
With regard to the SI lights, I regard them as indicating the latest time at which an oil change should be made, and prefer to do so twice-yearly, at about 8000 mile intervals.
From the 1996/97 BMW Fast Facts book. Under BMW features, all models, as written.
The SII microprocessor of BMW models to date received inputs from the starter, tachometer, odometer and an engine temperature sender; on the basis of these signals, it calculated the mileage when maintenace would be required. In the 1996/97 models, the microprocessor's programming has been simplified; it now considers only the vehicle's fuel consumption, which BMW engineers have found is an adequate predictor of service needs (higher fuel consumption = harder use). With either logic, the SII display in the instrument cluster indicates the time for maintenance. Owners should follow the recommendations of the SII and ignore anyone who recommends servicing at fixed intervals.
With regard to the SI lights, I regard them as indicating the latest time at which an oil change should be made, and prefer to do so twice-yearly, at about 8000 mile intervals.
From the 1996/97 BMW Fast Facts book. Under BMW features, all models, as written.
The SII microprocessor of BMW models to date received inputs from the starter, tachometer, odometer and an engine temperature sender; on the basis of these signals, it calculated the mileage when maintenace would be required. In the 1996/97 models, the microprocessor's programming has been simplified; it now considers only the vehicle's fuel consumption, which BMW engineers have found is an adequate predictor of service needs (higher fuel consumption = harder use). With either logic, the SII display in the instrument cluster indicates the time for maintenance. Owners should follow the recommendations of the SII and ignore anyone who recommends servicing at fixed intervals.
