A lot of work ( and I mean A LOT of work ) has been done on this very issue under the guise of the European Cabin Air Quality Initiative. Many accurate measurements have been taken on flights, targeting where possible aircraft that have had a reported history of oily smells.
The results are to be published shortly - don't know when I'm afraid. Preliminary findings were quite encouraging in as far as when an oily smell was present, the measurable nasties content was surprisingly small. I can't quote figures, but it was suggested that the air coming into your car whilst driving along a motorway was of a poorer quality.
Keep a look out for the report in a crew room near you. In the meantime, keep reporting the smells, otherwise you may miss out on any possible compensation payments when BALPA get their hands on it ( tongue in cheek ).