96 chevy k3500 vortec has oily residue underneath throttle body

truck has a chuggling issue on acceleration. also feels like it has a slight misfire at idle. took off the iac to clean it, then removed the air intake to clean the throttle plate. once the t-plate was fully opened, i looked inside and boy howdy. everything inside the intake was covered in a thick oily residue that couldnt be removed by spraying carb cleaner on it. is this simply a pcv issue, or could it be caused by blowby, or even worse, a leaking intake gasket? thing is, my dad replaced the intake gasket a couple of years ago. what are your thoughts, cartalk?