ΔE 14.8, fair: the same colour family, but not a straight swap. Mechanicus Standard Grey is a little lighter than Oak Brown.
Can I use Mechanicus Standard Grey instead of Oak Brown?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Mechanicus Standard Grey is a little lighter than Oak Brown.
Can I use Oak Brown instead of Mechanicus Standard Grey?
The same difference applies the other way round: Oak Brown is a little darker.
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.