ΔE 23.5, loose: different enough that you should pick on purpose. Oblivion Black Wash is noticeably lighter, much more muted than Mortarion Grime.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Oblivion Black Wash is noticeably lighter, much more muted than Mortarion Grime.
The same difference applies the other way round: Mortarion Grime is noticeably darker, 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.
See Mortarion Grime on the colour wheel See Oblivion Black Wash on the colour wheel
All matches: Mortarion Grime equivalents · Oblivion Black Wash equivalents