You should probably use synthetic since it gives you better fuel mileage and longer intervals between oil changes, and therefore reduces America's dependence on foreign oil and reduced the chances of old oil ending up in a landfill.
Which motor? The in-line 5 cylinder? In any case, just stick to The List of approved oils. I use Mobile 1 0w-40 because a) it's on The List and, b) it's the only one specifically mentioned by name in the Technical Service Manual, and c) because it was the initial fill at the assembly line.
I'd recommend picking one and sticking with that forever in that motor, though, as changing brands and viscosities will cause the engine to wear juuuust a little differently with every change.