Polymer will adsorb oil and hold it as a film on the external surface.
Steel (raw, unfinished steel) will gradually absorb, take in or soak up the oil by physical action. Depending on the type of finish (protection) done to the steel, it may adsorb oil or in the case of your "parkerizing" or phosphating, the oil is absorbed into the parkerizing.
A fingerprint is body oil adsorbed to the surface of your firearm.
A skid mark is absorbed into your skivvies.