Rocker box filling with oil

Hi There, Maybe somebody can shed light on this problem. I have a 1991 Nissan Micra 994cc 75000 miles. Symptons - Steam out of exhaust when engine hot,immediately on starting engine the rocker cover fills up with oil,revving the engine blows it out of the filler cap. No blue smoke from exhaust. My diagnosis is a blown head gasket,but why the oil in the rocker box? Help Please Regards Colin