arcniz ,
Good idea about sampling the air; somehow I think this may already have been done.
However there would be nothing to prevent investigators setting up a control experiment where oil is either deliberately put into the packs in various amounts ( with no pax - obviously) and the crew on oxygen and then test the air which has been polluted with harmful oil vapour. It would then be a case of persuading some pet guinea pigs to join the experiment to see how they faired.
I can't see this happening though as the results would be kind of obvious and predictable; it may proove to obvious a link to what is going on.
Then what would the authorities do?, a commercial nightmare or what.
As with all the previous uncomfortable academic research into contentious aviation findings it would be covered up instantly as being too complicated. The only thing the authorities are rather good at is, actual accident investigations.
First, you have to have an accident.