See if you can dig up something on a Shell turbine oil called ASTO 560.
This oil coked up another RR Trent around 2002.
see:
http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20040528X00693&ntsbno=DCA04IA002& akey=1
It was withdrawn by RR. the oil that is..
I checked and this oil is still widely available, maybe Qantas are still using it. Who knows?
Anyway the bottom line is it was coking the vent tubes from the bearing.
The most significant part of coking in a jet engine is hot air mixing with the oil. Leaking hot air into a bearing compartment through the seals leads to coking in the scavenge system which leads to higher pressure in the bearing compartment which leads to oil blowing out through the bearing compartment seals and possible light off around a spinning disk.
There is also the possibility that seals worn badly enough could leak oil out of the bearing compartment even though no excessive pressure rise within the compartment has been caused by coking in the scavenge lines.
Other possibilities exist so perhaps when the full details of the RR fix become known we can better understand.