It takes quite a bit for the USPS to open any packages. Usually, they have to be tipped off about some illegal activity. Then they x-ray the package and determine if they need to open it or not, most of the time they don't. The bad part is, if the USPS is watching a person for some illegal activity, they won't always bust the first package suspected as illegal substances or goods. They will watch and record the quantity of packages and their contents. As for being oily or leaking, they almost never open those packages, only if the package is soaked so bad that they can not read the address. Usually they will put it in a plastic bag and deliver it.