That seems normal for a diesel . The X5 has an exhaust gas recirculator which can become very fouled with carbon .
I read an interesting thread on another forum discussing adding 2 stroke oil to the diesel at a ratio of 200/1 as a lubricant for the high pressure pump .
Apparently hp pump failure is common since modern diesel has low sulphur content and no longer lubricates the pump .
Extensive testing by engineers at a Stuttgart based company who put a three pointed star on their cars found 2 stroke oil had many benefits including keeping the engine spotless inside after disassembling test engines after many tens of thousands of miles .They also run smoother and quiet .
Just use the cheapest brand of 2 stroke oil . Expensive synthetic stuff is a waste of money and not recommended .

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