Why shock absorbers leak oil, the risks of driving on with a leaking damper and the steps to take.
A shock absorber damps the free oscillations of the body after an obstacle. A healthy unit is sealed by the gland between rod and body; the seal is never perfectly tight, though, so a very slight film of oil can be normal.
According to MSG Equipment, the most common cause of leaks is excessive suspension load: under sudden compression the fluid cannot pass through the valves quickly enough and escapes through weak spots in the seals. With time or high mileage the seal wears and the damper loses its ability to control body movement.
A car with leaking shock absorbers should not be driven on: braking distances grow, cornering becomes harder and ball joints, knuckles and bushings wear faster.