Leaking fork seals are a sign of problems with the front suspension on a motocycle. The forks contain springs and a viscous damping agent, oil. The suspension's properties are altered if the springs or damping are changing in any fashion. If enough oil is lost, the front suspension will begin to 'pogo-stick'. Instead of compressing and extending in a controlled fashion (damped), it will compress more easily and will rebound suddenly, just like any undamped spring. A driver can lose control of the motorcycle far easier, especially on rough pavement, if the damping is lost. A very thin film of oil on the forks is normal, because the seals do not remove or 'scrape' off all of the oil from the surface of the fork as the lower tube slides up and down. However, if the oil begins to drip or collect (pool) around the seal, it indicates a problem that should be corrected. Replacing the seals may require special tools to disassemble the front forks, unlike performing maintenance on or modifications to the springs. It should also be performed in a clean environment, because if any dirt gets into the fork internals, it could cause problems (e.g., damaging seals, blocking ports open or closed in the dampers). You do not want the front suspension to pogo while riding over bumpy pavement in a curve, for example.

Hiya, also the fork oil can leak down the fork legs and get onto the brake discs. not so good!