I had that problem Lee. It is the oil seal on the motor.
It was down to me not cleaning the area around the oil seal and eventually the grit wears the seal out.
That was about 2 years before I sold it, les fixed it. You will need to get the motor into him.
It is an area you do not think of washing out. We live and learn.
Forgot to add? You can put new oil in meanwhile, there is a oil level plug.