Use what is recommended in the Owner's Manual. If you follow the intervals described there, you don't need anything special.
Many of us use Mobil 1 and follow recomended change interval while under the 36 month warranty and then go to extended intervals.
ive used synthetic on all my vehicles for several years now. my mercedes engine warranty is voided (im not kidding ) if synthetic is NOT used. once the car is thoroughly broken in i cant think of reason not to use it aside to save a couple of bucks. as far as viscosity goes i always do what the manufacturer recommends,