being there is not a dip stick in any crossfire ask them what the dip stick read ?
the dealer has a universal dipstick that works in most Mercedes, the oil level is read from the bottom of the pan up. and yes oil can cause that problem if they can stop the leak and proper for the amount of fluid in its it's needed,, it also takes a special fluid. I'm not sure which one but it is a special fluid that Mercedes uses.
if they insist on the overhaul. I would ask them to To fix the leak and the only way they're going to find the leak is to put a bullet in it once a path to eliminate you should drive it and see if it's working okay if it is most likely nothing is wrong with it don't drive it and just drive drive it in the slapstick also .. If it that time it's okay then have them repair the leak bill it back up with polluted and you're ready to go good luck