My understanding (given without prejudice) from chats I've had with engineers, is that toxic fume events are more likely to the product of oil seal design on the C5 engines, and the fact that damaged seals are not replaceable by our own engineers.
Thatīs right Sapco, you canīt get at the Labyrinth seal on any multi spool turbine without a lot of surgery.
http://en.wikipedia.org/wiki/Labyrinth_seal
You can imagine one of these between each set of spools as well as at the front of the engine. Each spool has to be disassembled from itīs shaft to gain access to the seal seperating it from itīs neighboring seal, and each seal itself has to be disassembled. Itīs a very labour intensive and expensive job to get at them. Basically youīre talking about a complete stripdown to replace them.