synthetic/blend/regular oil.... which is best to use?

I just purchased my 2003 350z two weeks ago, and I need to change the oil since they didn't before I bought it (kind of a blessing in disguise if you ask me). The oil was perfect, no contamination in it from other fluids, but it was a bit dirty so I realized they didn't change the oil, but since I didn't buy it from a Nissan dealer, I don't think they'd have used the right oil for it anyway considering the cost of synthetic. What oil does everyone on her use in there 350z's and does brand matter when it comes down to synthetic if synthetic is the choice?