If your vehicle is showing greasy fluid leaking down the shock absorbers or struts, here are the most likely causes and the parts you'll need to fix each one.
Over time, the internal hydraulic seals in your shocks or struts degrade from normal suspension cycling. When they fail, the internal dampening fluid leaks out and coats the outside of the housing.
If the protective rubber boot tears, dirt and road grit stick to the polished metal shaft. As the suspension moves up and down, this grit acts like sandpaper and destroys the hydraulic seal, causing a leak.
A severe impact from a deep pothole or curb can over-pressurize the shock or strut, instantly blowing the internal hydraulic seal and causing a rapid fluid leak.
If pliers or vice grips were used to hold the polished metal shaft during a recent installation, the resulting scratches will immediately tear the new seal and cause a leak.