Dirt, mud, sand, metal shavings, or other abrasive materials: scrape and gouge internal surfaces. This allows a path for air or oil to escape. You may see oil running down your fork legs or a loss of air pressure. Long term exposure erodes tolerances on bushings, stanchions, and seals. This causes play between stanchions and lowers and less control over front wheel. Contaminated oil also clogs oil ports, which effectively increases compression damping. This reduces small bump sensitivity, makes for a harsh compression feel, and limits your ability to get full travel.

Water: mixes with oil when in motion and changes viscosity. Water naturally settles to the bottom when still so viscosity is unpredictable. Viscosity will be decreased with water, which causes faster compression and rebound.