A petroleum- or silicon-based oil that is available in different degrees of thickness, used to fine-tune the damping of a shock absorber As the shock shaft travels up and down, the piston moves up and down also, so the oil must flow through the holes in the piston The smaller the holes are in the piston, the slower the piston is able to move, and with larger holes the piston is able to move faster When you are starting out racing and you want to try adjusting your shock settings, you should try changing your springs first, then your piston/shock oil combination