Nissan does not officially recommend synthetic oil for a few reasons.
1) There are many brands of synthetic oil out there. Most are perfectly fine (they have the API and starburst on it). However, I'm sure those miracle cures (that don't work) like Slick50 and Duralube are also synthetic formulas, and I can see why Nissan would not recommend those. You would be surprised how naive many owners are, and they think that stuff is "synthetic oil" in the league of Mobil One or Castrol Syntech.
2) Using conventional oil means more frequent oil changes, leading to more $tealership visits, leading for more opportunities for them to $crew you over with unnecessary $ervices.
But BMW and Porsche come standard with Mobil One you say. Well those owners are also used to paying $1000+ for normal service intervals whenever they go in, not something your average Nissan owner will likely enjoy.
2003 Midnight Blue SE AWD w/VDC
Colgan Bra, Mobil One, JWT POP charger
PAC aux input adapter, hardwired Escort Passport x50, KUDA mount for MP3 player, Porterfield R4S brake pads (now Akebono ProACT), ATE Superblue
Yokohama Envigor (much improved over stock Goodyears)
Lots of squeaks and rattles.