acetone and a scrubby or steel wool will lift all dirt, grime and residue out of the top layer of wood grain, without harming the original color of the oxidized linseed oilicon or damaging the deeper original sanding. Simply wipe it down with linseed oilicon when you are done to restore the color and finish. I just did this on a Kragicon that used to hang on a wall at a VFW for decades...imagine decades of cigarette smoke from the vfw bar (it reaked of smoke as soon as I wiped it down the first time with mineral spirits), combined with decades of sweat and dirt from parades etc. I posted the results, with before and after pics, in the Krag forum.
https://www.milsurps.com/showthread.php?t=71678&page=6