I to had this similar problem......the rear intake bolt had lost torque and allowed the rear of the intake manifold to lift and cause a leak of
antifreeze into the
oil.....my
oil looked like a milkshake but as you said in your case it only shows on the inside of the rocker covers.......if the leak is that minute it will be very hard to detect.....i say what Patrick said......put the other manifold back on and run it and see what happens....and if i paid some hard earned cash for a new block and manifold and they leaked...someone would be getting them shoved up the their tailpipe.....isnt that why you buy new stuff so you dont have problems like this.....