It doesn't just have to do with BMW offering free maintenance. The used fluids and filters are tacked on in regards to how environmentally friendly cars are. By increasing the mileage between services BMW effectively makes their cars more environmentally friendly as far a rating agencies are concerned.
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2010 BMW 135i AT, AW, ///M Sport
JB4|BMS DCI|MS 3" cattless DPs|Golf Tee|33% Tint|///M Pedals