It depends on the oil and your use. Not all synthetic oils are the same, and not all conventional oils are the same.
Personally I use an amsoil product called OE which is their cheapest oil, it's a synthetic, and I change it every 5k miles.
Also, there are no sludge issues whatsoever with the 2008+ 4.7L.
The owners manual in my 2010 4.7L calls for 6000 mile intervals, with conventional oil, IIRc. BTW.
Breathing in and out is good enough for me.