1989 560sel -help!!

Just started - when car started from cold - no problem - as soon as temp guage hits 85C - small plumes of white smoke come from under hood on passenger side. Most noticable when sitting at a red light. Upon lifting hood - you can smell burnt engine oil and it appears to be burning off the exhaust manifold on passenger side BEHIND the head. It is behind the engine head near fire wall. Problem gets worse on right hand turns. What we can see of exhaust manifold is dry -no sign of oil so it is behind the head where the leak is. Everything else is normal. My mechanic said it was a leaking valve cover gasket. Replaced it with a new one - no difference - when car is cold no oil smoke - as soon as it hits temp (85C) it starts burning oil off the back of passenger side exhaust manifold behind head. I thought maybe head gasket but my mechanic says it is below exhaust manifold and cannot be the problem. Any thoughts before we start looking??? Can the valve cover warp? - bolts are torqued properly. Gasket installed properly. When engine is not yet at temp. and exhaust is hot - no smoke!