Ditto What TX5 said above. You can pretty much get a good understanding of the "BMW recommended Oil Serv/Inspection" interval and what should be done by the link posted by pezho405 in post #4 which shows the old SIA I & SIA II schedules that were used when
mileage & engine load were 2 of the main considerations for service intervals.
On your vehicle, BMW has gone to SIA III where the SI interval is determined by fuel consumption, which is different for each engine/model designation. The old "mileage" determination is still not far off from the "fuel consumption" determination if all things are "average". But if they are not, then you can have a shorter/longer interval depending on the individual driving habit/style you may have.
With all the above said, you can still use the SI display to indicate when you may want to do an OIL SERV/INSPECTION by using when the remaining amount of lights illuminated to determine when its "your" time. If the 12k-15k mileage interval is too long for your taste, then it can be assumed that each green light indicates approx
2.4K-3k miles. Or you can still look at it through the prism of fuel consumption and change the oil when 2~3 lights have gone off when you've consumed
40%-60% of the pre-determined fuel consumption for you model & engine.
Again, our vehicles now use SIA III which was introduced with the 1996 e38 7 series...and now is used on all BMW models: