Blue Tone vs Nuln Oil

How close are they?

ΔE 35.6, loose: different enough that you should pick on purpose. Nuln Oil is noticeably darker, much more muted than Blue Tone.

Can I use Nuln Oil instead of Blue Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Nuln Oil is noticeably darker, much more muted than Blue Tone.

Can I use Blue Tone instead of Nuln Oil?

The same difference applies the other way round: Blue Tone is noticeably lighter, much 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.