Oily Steel vs Dark Gunmetal

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Dark Gunmetal is noticeably lighter, more muted than Oily Steel.

Can I use Dark Gunmetal instead of Oily Steel?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Dark Gunmetal is noticeably lighter, more muted than Oily Steel.

Can I use Oily Steel instead of Dark Gunmetal?

The same difference applies the other way round: Oily Steel is noticeably darker, more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.