1996 Subaru Oil residue in the cooling system expansion tank

I have noticed a dark oily residue in the cooling system expansion tank of my 1996 Subaru Outback. It also smels like burned oil. I have been noticing this for the last year or so, but the repair shop says it is no problem, and that this is the way the cooling liquid degrades. They have flushed the cooling system, but a month latter the oil residue is back and the smell too. The repair shop says they did not noticed a softening in the hose on the top of the radiator, so there must not be any oil leaking into the cooling system. Any thoughts?