oil in spark plugs why is this so and how bad is it

If the plugs have oil on the electodes, you will eventually get a missfire. Missfires will damage your catalytic converter. If you have oil in your spark plug holes throuth the valve covers, this will also cause m,issfires and will also cause spark plug wire/coil damage. Most common cause of oil fouled plugs (oil on electrodes) would be valve seals or guides