It can be one of two things. As described earlier it can be the seals in the rocker cover that go hard and leak. Just replace them and it will be o/k. The other common cause is the spark plug tubes leaking where they screw into the head.These are the metal tubes that the plug leads push into. Carefully unscrew them with a pair of multi grips trying not to crush them. When you get them out clean the threads and apply silastic to the thread and screw back in. I have done heaps. It will work every time.