Croqueteer,
I would like to know the real reason that Boeing have gone back to and independent supply of cabin air.
The B787 engines are "bleed-less" to improve the efficiency of the engines while reducing the weight, valving and piping complexities of both engine and aircraft, all of which reduces cost, the exception being those bleeds required for engine heating/cooling purposes. The traditional turbo-fan engines (two spool design) take air out of the compressor, usually in three locations. BOAC is correct, none of the removal location are in the turbine area.
For example, in the CF6 engines, bleed air is pulled off at the 7th, 8th and 11th stages of the high pressure compressor. This is pretty far back in the compressor and the air drawn off is warm. The 8th stage bleed air is what is provided exclusively to the aircraft for various purposes including cabin air.
So how might oil fumes get into the cabin air? Well, some air is drawn off the fan by-pass and is used for various reasons including pressurizing the "B" sump that sits in a compartment underneath the combustor forward of the HPT. The excess pressurization air is then sent forward and vents into the cavity area where the fan bearing and stub shaft joins the shaft connected to the LPT. The air is distributed through several compartments to eventually heat the shaft to maintain seal clearances.
Now as the seals begin to wear over engine operating time, some oil in the form of mist could possibly escape into the main stream airflow into the HP compressor and be drawn off at the 8th stage bleed into the cabin air. It would probably only happen in certain engine transient modes.
All engines are different in details so this is just one example and one possibility. It does seem to me that the chemical makeup of modern oils used in engines is important regarding air contamination and what crews (and passengers) are subjected to from time to time.