Bottom line is that you get more/better protection from synthetic. The guys at the stealership probably don't have synthetic as an option, hence telling you not to do makes it so you keep coming to them for simple oil change and maintenance.

Personally, i'd recommend doing what i'm doing. I'm gonna let the stealership do my first oil change (it's free and I wanna make sure there's nothing else to check on). Then, when i'm ready for my second oil change @ 6K, i'm takin' it to EZ Lube and having them put in Mobil 1 full-synthetic. It doesn't void the warranty and it's just better performance.

Then, when my warranty kicks, i'm switching it over to AMSOIL full-synthetic which only needs to be changed @ 25K/1yr and the AMSOIL filter @ 12K/6mo. If you do service intervals like that during the warranty, it gives the stealership and Nissan easier ways to void your warranty if something is wrong.

B.