I recently bought a 2010 Mustang GT (still waiting for delivery, as it has to be built since I ordered it with many options), and thought that I'd offer a few pointers.
I've done some extensive research on the topic of oil, since obviously I'm a BITOGer, and I only want the best for my car.
The GT, as it may come as a surprise, doesn't specifically require synthetic oil. If you go to any Mustang forum on the 'net, you'll be bombarded with replies about using Mobil 1, Royal Purple, Pennzoil Platinum, etc. It's somewhat of a misconception among Stang owners. I'm not necessarily knocking this viewpoint, but just wanted to point out that it's not mandatory to use synthetic with this car (unless it's the Cobra, but that's another story). Just check the owner's manual, and you'll see that Ford recommends a 5W-20 that meets their M2C930-A spec for the 4.6L V8 engine found in the GT.
I, on the other hand, will most likely run conventional oil in my Stang, when it will be delivered later this month. I'm going with 5W-20, as this grade is recommended by the owner's manual. What specific brands, you might ask? Pretty much all of them, as long as they meet the M2C930-A spec for Ford (my favorites are Havoline, Exxon Superflo, and Formula Shell). I'm also exclusively using Motorcraft filters, as I feel comfortable using them since they're designed and recommended by Ford.
I might take the car to the track from time-to-time, so in those occasions I'll definitely dump the dino oil and go with a synthetic for extra protection. Obviously I realize that your use will certainly be more demanding that mine, so you would benefit more from a synthetic than someone in my situation (daily driver, commutes 40 miles a day to work in stop-and-go traffic). I thought that I would simply offer another perspective for other Stang owners who drive in conditions similar to me.
By the way, I change my oil every three months regardless (I put on about 3,500 miles in that timeframe), so I'm not worried about extended drains or any build-up in the engine.
Hope this helps, and let's keep the American muscle tradition strong!