The phrase "oily residue" refers to a thin layer of oil that is left behind on a surface after something oily has been in contact with it. Full definition
The soil can be applied to your skin without leaving oily residue. (geekwrapped.com)
No more oily residue with topical flea / tick treatments. (1800petmeds.com)
It's different than traditional, password - based approaches, which validate passwords instead of legitimate users, as well as behavioral biometrics - based solutions, which typically involve touch screens, fingerprint readers or other costly hardware and lead to privacy concerns and «smudge attacks» that trace oily residues on surfaces from fingers. (sciencedaily.com)