When changing my spark plugs the other day I noticed some oil on the rubber boot of the coil pack on the rear passenger side cylinder (1 or 4, I believe). It looks as though the o-ring that seals the spark plug area is leaking. How serious would it be to leave this for a while?

 

Not very serious usually, but very easy to fix. The danger is oil dripping on the exhaust and starting a fire but it's pretty remote. The o-rings have been updated. You can usually pull the spark plug tube out with your fingers (no need to remove the plug) and replace with a new tube and new inner and outer o-rings.