It has to be, to keep bearing races rust-free. Petroleum attacks most plastics, and over time can warp or even dissolve them. Rubber or plastic parts must be removed from the firearm and stored where oil, grease and petroleum vapors from these products cannot affect them. This brings us to wrapping the firearm and its parts for storage in the container. You'll notice that the military doesn't use plastic bags for petroleum-lubricated parts storage.
Plastic is no good because oils dissolve it.
The government uses an expensive treated paper which alone is a rust inhibitor. You can use kitchen-grade wax paper. Once your firearm has been packed in grease, wrap it completely in several layers of waxed paper. Leave no openings to admit moisture, just in case your container suffers a small leak. The foil will block the petroleum vapors which can attack the plastic and rubber parts.