I own a 2005 Altima SE 3.5 with 5-speed auto. It's got 48k miles and I decided I want to replace my ATF with a full synthetic blend. I was wondering what the best option would be.

Is Amsoil synthetic ATF safe for my tranny?

From the Amsoil website looking at their universal synthetic ATF:

"AMSOIL ATF is recommended for transmission, hydraulic and other applications requiring any of the following specifications:

Nissan Matic D, J & K "

Double check what the book recommends for that think (it would seem like Nissan Matic K from what I am reading but don't quote me on that as I am new at this), and as long as it is one of those three formulations then you're good.