2002 Buick Century Oil in Antifreeze

While driving recently, the low coolant level light came on. The radiator had oil (sludge) in it. The car has been running fine. The mechanic I had look at it said this model of car is known for having intake manifold gasket problems and stated this is the problem. The oil looks fine. Could this be a head gasket problem? If it is only an intake manifold gasket problem, would it be worthwhile to also do the extra work of replacing the head gaskets (e.G. Is this car notorious for head gasket issues, also?)?

Thank you.