I have a cheap cartesian printer with the usual arrangement - X axis with three linear bearings, build plate moving as Y axis on three linear bearings as well, Z axis with lead screws. After about 2 kg of filament printed, I can see that one of the linear bearings (the single bearing on one side of the Y axis) is leaving an almost black residue at the front of the axis. I have thoroughly oiled all of the bearings before mounting them and repeated that after about 1 kg of filament printed. No other bearing is showing this issue, and the black goo appears only on the front side of the rod, not the back side. What might cause this goo - is the bearing breaking up or is it just some residue from the manufacturing process that is slowly being pushed out of the bearing?