engine smoking?

Does it smoke at idle and shoot out a big puff when you pull away - most noticeable at stoplights and drive-throughs? That's the classic signs of bad valve seals. If that's the case then they can be replaced. You can do it yourself if your comfortable doing engine work but it is a pain in the butt.

If it's smoking all the time or at WOT then it's more likely that it's worn rings and a rebuild is in your future.

Check the PCV system out first though as a problem there can cause excessive oil consumption too.

Forgot to add that valve seal replacement can be pretty cheap if you have the tools, time and skills to do it yourself - $20 for seals, $30 for the Trick Flow Specialties OHC valve compressor tool, save a few more $$ if you can reuse your valve cover gaskets, borrow or rent the other special tools you need. Not sure how much a shop would charge but it could be really expensive if they wanted to pull the heads to do it.

Mustangs consume some oil, especially if you spend a lot of time at WOT and high RPMs so don't be alarmed if it still uses some oil after you get it fixed.