Good question. I'v had my GN for about 3 years now and I always use Castrol Centec 10w40
Well i was told by Ron from Rons custom auto to use 2 quarts mobil 1 synthetic and 3 quarts mobil 1 standard 10w30 i wasn't sure about mixing up synthetic and standard but he knows what he is talking about and i feel the cars is running perfect so thats what i use
I have been using Mobile1 Synthetic in all my cars (GN included) forever. I tried Royal Purple in the GN this year. Didn't notice any performance gains but didn't really expect to. Just got a good deal on it so I gave it a whirl.

I have run up 170k miles on my Daytona with Mobil1 its whole life and the oil pressure is a strong as day one and it doesn't blow/burn any oil -- which is a Crysler/Dodge trademark on 80's cars :)
Valvoline 10W30 full Synthetic. Any good sysnthetic,
just pick a brand and stick to it. When you first
put in full synthetic, it will find every oil leak.
And then you have to fix them or live with them,
but your engine will be happy.