This is an educated guess type of reply, rather than scientific but ... I have often cleared 'film' (i.e. grease, some dirt, or caked condensation, etc) off coins by immersing in surgical spirit and then wiping that off GENTLY using a microfibre cloth. I wouldn't use olive oil for what you plan to store in a plastic envelope, though a very thin film of it might do for coins in trays, but make sure you reapply every now and then - either annually or when you judge it's needed. But if olive oil is reportedly 'mildly acidic,' wouldn't a continuous film of the stuff eventually have a negatory effect? I seems to vary from manufacturer to manufacture. The stuff we have at home at the moment seems to be completely neutral