I used 5w-30 and 10w-30 Max Life Valvoline in my 95 Grand Cher. Jeep. The head gasket leaked before using the Max Life and after about 200 miles, the leak was gone. I have since switched to Chevron Supreme and still no leak.
Unfortunately using a thicker oil isn't going to help an oil leak in the cylinder head of the Triton motor. That is a result of poor machining and the gasket used. What Ford is supposed to do is check the cylinder head and block for "straightness" or eveness. And if it's in "spec" then use the new improved head gasket. Otherwise they replace the head or block, depending which is out of "specs". Not cheap any way you look at it. Sometime the dealer will go to bat for you with Ford and at least split the costs so you aren't stuck with the full bill. It's worth a try, especially if you've dealt with this dealer for a while. Good luck.