First of all, congrats on the new car! The car is a beast!
I run Amsoil Signature Series Multi-Vehicle Synthetic Automatic Transmission Fluid for all my vehicles that meet the requirement (Honda CRV, Mazda CX5, Mazda 6, Honda Accord and Toyota Camry). Out of all the vehicles, Honda transmission is probably the most "picky" because Honda strictly states in its manual that owners should use ONLY Genuine Honda ATF-Z1 or DW-1 (whichever your vehicle calls for). Honda issued a major recall in 2004 for Honda Accord transmissions due to early and unexpected failure of the transmission. As luck would have it, I own a 2003 Honda Accord with 80k miles that did not fall under the recall. I had the transmission rebuild last month and I used Amsoil ATF along with an in-line transmission filter for peace of mind. I also run Amsoil ATF in my Honda CRV and I can tell you that both the Honda CRV and Honda Accord shifts smoother and engages faster compared to the Honda ATF-Z1.
I cross-referenced your vehicle on the Amsoil website and Amsoil Signature Series Fuel-Efficient Synthetic Automatic Transmission Fluid meets the requirement of Nissan Matic-S and Matic-W. Although I've never used Amsoil ATF in an Infiniti or Nissan, I'm sure it'll work just fine.
As far as the rear differential goes, I use Amsoil Severe Gear 75W-90 (GL-5) on my Mazdaspeed Miata. I can't say I noticed much difference after switching from factory diff oil to Amsoil but it gives me a peace of mind that the fluid can be in the differential for at least 50k miles under severe service. Your Q50 rear differential calls for 75W-90 (GL-5) weight.
If it was my car, I would flush out all the fluid and switch to Amsoil. It is a great product (expensive) but gives me a peace of mind!
I hope this helps.
-ttvr4