Oily Steel vs Necron Compound

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. Necron Compound is a little darker, more muted than Oily Steel.

Can I use Necron Compound instead of Oily Steel?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Necron Compound is a little darker, more muted than Oily Steel.

Can I use Oily Steel instead of Necron Compound?

The same difference applies the other way round: Oily Steel is a little lighter, 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.