If it helps I have also recently noticed oil in the spark plug tubes on my 2005 gt and found the cause was the cam cover O rings failing on the passenger side, middle and rear cylinders. After scouring the net I found the O rings are laminated in to the cam covers and cannot be replaced individually without choppin the cover up and importing them from US so I rang the stealers in Warwick to get a new cover and they said £248! excluding main gasket which is extra £28 or sumthin. I've opted for RTV for now for a cheaper fix, I'm sure it will work. I use it at work a lot
You wont loose litres through your spark plug tubes so high oil consumption wud probly be internal related. id get the cylinders pressure tested and leak down tested to check condition of piston rings and valve stem seals.