ΔE 15.5, loose: different enough that you should pick on purpose. Mechanicus Standard Grey is more muted than Medium Russian Green S-34092.
Can I use Mechanicus Standard Grey instead of Medium Russian Green S-34092?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mechanicus Standard Grey is more muted than Medium Russian Green S-34092.
Can I use Medium Russian Green S-34092 instead of Mechanicus Standard Grey?
The same difference applies the other way round: Medium Russian Green S-34092 is 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.