Nuln Oil vs Umber Wash

How close are they?

ΔE 42.2, loose: different enough that you should pick on purpose. Umber Wash is noticeably lighter, more saturated than Nuln Oil.

Can I use Umber Wash instead of Nuln Oil?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Umber Wash is noticeably lighter, more saturated than Nuln Oil.

Can I use Nuln Oil instead of Umber Wash?

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