[recce]

During my days driving a WRX, it was recomended by Subaru that every 10,000km, the engine will need to be treated with subaru's "Top engine cleaner". Because all boxer engine will have carbon built up quickly, the "engine cleaner" will clean the scams and carbon built up within the intake manifold. It cost about $14 bucks from subaru and all you need is to spay it into the manifold via one of the vacum tube while the engine is running. You then stop the engine for about 10-15min and re-start agian. This time, take the car for a good blast and it will clear all scam and carbon built up in the intake manifold.

Not sure if BMW have such product on their shelf