it sounds like you found the problem...providing the oil drain plug passes inspection. and there's no cracks in the case(s).
never heard of oil eating a gasket before
...you would think it would be eating up the seals too if that were the case.
Get a Cometic gasket to replace the old one....they're a rubber-ized coated alloy gasket that really seals things up nice, and they are re-useable.