10holelx is right. Also, aftermarket intakes will force more oil up there. I noticed you have a TFS intake. Don't know much about that, but the Ford GT-40 is known to make this problem worse. Of the three Mustangs we have, the two with the stock intake and regular oil never need oil added. However, the one with the GT-40 and synthetic oil needs to have oil added almost every 1,000 miles.
I tried one method of reducing this and have found some success. You can remove the vacuum line that goes from the oil neck on the valve cover to the throttle body. Plug them and run a breather.