"Oils smells from the APU can also come from people not following the securing checklist properly. The batteries shoud only be switched off when the APU door is closed. Do it before and oil from the APU can be sucked into the ducting (not sure why) giving an oil smell when the APU bleed is on. This is of some concern as engine oil fumes are toxic."
"Reference please? Mine is the Airbus AMM and TSM (Air maintenance and trouble-shooting manuals)"
FCOM 3.03.26 p1. Securing the aircraft:
BAT 1 and 2........................................................... .....OFF
Wait until the APU flap is fully closed (about 2 minutes after APU AVAIL light goes out) before switching off the batteries. Switching the batteries off before the PU flap is closed may cause smoke in the cabin during the next flight.