There is nothing wrong with using either synthetic or conventional. In fact, most SN conventional is actually a syn blend anyway.
Pennzoil's Dexos Blend should work well in that application, as dexos is designed with DI and turbo in mind. the Pennzoil blend is at least 50% syn.
Another alternative is to look for cheap $20 synthetic oil specials, like the Havoline syn that AAP had last month, or the occasional Napa sales they have on their house brand synthetic (which is rebranded Synpower).
I would stick with Nissan's OCI recommendations while under warranty.