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The amount of fuel you may save depends on your driving pattern. In the Double European Cycle currently used in Europe, S/S supposedly saves 3%, but on the US EPA cycle, it saves nothing. The European cycle has many stops of between 30 and 90 seconds duration.
The X3 does apparently have a high-capacity battery and very high-capacity alternator. Hopefully, the starter motor is also heavy duty, but it appears to be the same one used on versions that don't include S/S.
There is no doubt that using S/S must result in additional wear to the starter motor and starter drive, and possibly to the engine internals due to starting under reduced bearing oil pressure. How much fuel you can save is a matter of how much time you spend completely stopped in heavy traffic. To use it or not winds up being a personal decision based on the fuel you MAY be saving and how long you expect to own the car. If you trade in often, the extra wear probably won't be a concern. However if you want to drive the X3 for 150,000 miles and keep it well beyond the warranty period, it may be a major concern.
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