Solvent resistant is not the same as solvent-proof. As it happens I use something in my day job that dissolves pp; it's called solvent weld, I use it for plastic conduit that I'm running cables through, plumbers also use it.
Mineral oil contains a mix of alkanes and will dissolve pp, just very slowly.
You are probably correct that this does not cause the odour though, the effect would be too small and too slow.
The most likely explanation AFAIK is degradation (not decomposition) of the oil by oxidation which causes polymerisation (the opposite of decomposition if you think about it) and the evaporation of the lighter fractions.
My knowledge of organic chemistry isn't good enough to expand on that though, and I'm at work and using my phone to reply, so not in a position to go on an extended gurgle safari.