All suspension units require regular maintenance for them to perform their important role efficiently. Over time your suspension fluid / oil will pick up very fine particles of aluminimum, dirt and moisture through normal wear as the fork internals perform their role through all the compression and rebound strokes during riding.
This build up of dirt and fine particles is what causes the suspension fluid / oil to turn a dark grey colour. When the oil gets to this stage in it’s life, it begins to act more like a very fine abrasive paste, wearing the components inside the forks, in particular the bushings, which play the important role of reducing friction between the inner and outer fork leg and the internal damping rod. Worn bushings increase the levels of friction and reduces the performance and effectiveness of the forks or shock.To keep your forks and shock absorber in top condition and performing efficiently have them serviced regurlary, especially if you ride in the wet frequently.