hmm right change the oil and filter see how it goes if it doesn't clear strip the head down and check oil the valves make sure they are not burnt out replace all the oil stems seals and replace the tappets

is your blk top or silver top zetec ?
if blk top engine will have to re shim it as they are soild tappets
if silver top is hydraulic its a straight followed swop over but use proper assemably paste on the job too on both engines what ever you run also use a std ford head gasket and make sure get the head lightly skimmed