I would suggest that you conduct a search on this website for C280 head gaskets. You will find that this is one of the "issues" for this model.

Botton line is that if you can stand the leaking oil on your garage floor or driveway(leak is an external issue only), keep driving it. Retorquing the head will have little to no effect on the amount of leaking oil. The replacement gasket has been upgraded and once it has been renewed, it should not reoccur.