Many posts hinge on the proof of causal link. All this really demonstrates is that proving a causal link to legal standard for anything is hard. There was a similar rearguard action over agricultural organophosphates in 70's and early 80's relating to near-identical medical issues. Whatever the debate on causality we KNOW aero-engine oil contains OP's. Pushback is where I often experience oily smells. Would it not make sense to delay switching on packs until moving forwards?