Greetings... Just to shed some light on this issue. The fumes from synthetic turbine oil, such as AeroShell 560, BP2380, BP2197, MJO II, MJO 254 etc. are not toxic as commonly suggested. Now there is an extremely small amount of tricresyl phosphate (TCP) used in most synthetic oils, which if swallowed in large amount, is certainly undesirable. You literally have to bathe in these types of oils for a looong time to get a skin reaction...
Now I will not argue the fact that the odors can be uncomfortable and cause dizziness and headache, just like most strong odors... but not necessarily toxic. The problem you are describing, as mentioned above, is typically a leakage from the engine or APU oil seals, which allows for a small amount of oil to enter the compressor and eventually the ventelation (ECS) system. Additionally, engine manufacturers have different solutions to dynamic oil seals; such as carbon seals typically used on P&W and I believe also Rolls Royce engines?, rotating air/oil seals used by GE engines etc. Unfortunately the BAe146's Textron Lycoming engines have not completely solved this problem, and you can often get a good "whiff" of engine oil fumes during high power settings... but will eventually dissipate. Don't know about the newer AlliedSignal engines? A severe internal oil leak, where oil enters the compressor, will initially manifest itself by strong odors in the cabin and eventually high oil consumption. This is often recognized by a borescope of the compressor where “staining” will be evident and subsequently corrected as "ukeng" pointed out... Over servicing is another problem, where the engine has to “shed” the excessive oil through the oil sumps and overboard… but will again dissipate once the correct oil level is reached... I know this is somewhat simplified but should shed some light on this topic...
Dag