The pcv valve is a major culprit where smoke is concerned. It MUST be replaced by the BMW once the mileage is over about 80k or more than 5years old. (BMW have made it a service item and so too would MGR had they still been in existence.)
There are many variables, from the maf sensor, egr valve, pcv, air filter to the general state of the engine maintenance that can affect the smoke level.
The egr valve is proving a very unreliable and troublesome component on many diesels - eg Fords & Vauxhalls. Fortunately, its not relaibility but the inevitable clogging up that is the issue on the 75/ZTs engine.
Clearly no one can guarantee any particular smoke readings with all these variables but as we collect data, we'll be able to build a picture of what has most effect.
Ron
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