Nuln Oil vs Flesh Wash

How close are they?

ΔE 51.3, loose: different enough that you should pick on purpose. Flesh Wash is noticeably lighter, much more saturated than Nuln Oil.

Can I use Flesh Wash instead of Nuln Oil?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 51). Flesh Wash is noticeably lighter, much more saturated than Nuln Oil.

Can I use Nuln Oil instead of Flesh Wash?

The same difference applies the other way round: Nuln Oil is noticeably darker, much more muted.

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.