Once upon a time a dealer told someone the same story so someone had a new bmw engine fitted after spending fortunes trying other ways to cure the problem .
The new bmw engine made no difference whatsoever so someone got a bit pissed off at the dealer and eventually figured out the real problem was the crankcase ventilation system diaphragm had burst causing the intake system to suck lots of oil at idle filling the intake manifold .
You can test for diaphragm failure easily by removing the dipstick or oil filler cap to feel for excessive vacuum . This link will demonstrate .
How to change a dead OSV on a BMW E38 7 Series with an M62 V8 4.4L engine. - YouTube