1998 p, 2lt auto vauxhall vectra.. why is there oil?

in where my spark plugs go....? i took off the cover n took out my leads to cheak my plugs and the whole bay was filled with oil... i cleaned it all out cheacked the breather pipes they were all clear... but there is oil coming down the engine block from the rocker cover... could it be a failed gasket? or is it something else?