This shock oil can reach high temperatures on some tracks and actually burn into the shock valve discs. Note the discoloration of this disc in the shape of the piston.
The seal for the shaft can become scored and torn from heavy use and having to scrape dirt and grime off of the shaft as it moves in and out. This seal will need replacement often, especially in a dirt track environment.
This is the piston seal that lies just outside the O-ring around the piston. The O-ring forces this seal onto the inside surface of the shock tube to seal the compression and rebound chambers. This seal rubs on the inside of the shock body and is a high-wear item.
A typical shock piston is shown. Of note are the three dots impressed into the piston just outside each of the three flow holes. This represents a dish shape of standard one degree plus three (three dots = three more degrees) degrees for a four degree angle of the preload dish. The height of the face of the piston at the center hole is lower than the height of the face out at the edges of the flow holes (three outer holes) and this forms a dish shape.
Under the piston seal lies the O-ring that presses it onto the inside of the shock tube. These two components work together to form a seal that separates the two chambers of the shock.