The phrase "oily sheen" refers to a shiny or glossy appearance on the surface of something, like water or a substance, due to the presence of oily or greasy substances. Full definition
Process, scraping down sides occasionally, until praline paste is very smooth and creamy (it should be as smooth as peanut butter with a slightly oily sheen; this takes time, so be patient), 12 — 16 minutes. (bonappetit.com)
The fact that his buttocks were entirely hairless and had an oily sheen was something known only to those closest to him. (religion.blogs.cnn.com)
And it doesn't even take that long, plus there's no standing over a hot vat of oil, stinking up the house in an oily sheen for days! (loveisinmytummy.com)