Well, this oily substance was leaking on the inside, which means it's the evaporator coil.
Originally posted by billva
depending on the condition and safety of the furnace, you can keep it, if all is good.
the evaporator coil is a seperate entity as is the condensing unit (outdoor section).
the furnace only provides the air flow in cooling mode.
oil is definately a sign of a leak, and more than likely more to come in the future. a common place for leaks on a carrier coil is the threaded fitting on the liquid line (smaller) entering the evaporator coil.
it could be simple, and adding refrigerant while you decide sounds like your best bet.