I've got the same vehicle that you have and I love it. What I've done is to change the manual transmission fluid. I changed all the fluids not long after I bought it. If you drain and replace your rear diff fluid, you will see that it is quite dark and there is a kind of metallic paste on the drain plug. This is normal and is the result of wear on the metal gears of the diff as they wear in. I replaced the rear diff fluid with Mobil 1 Synthetic.
I also replaced the manual transmission fluid, but it looked very good, no darkness or evidence of gear wear. The one thing you need to be aware of though is that the manual transmission takes GL4 gear lube. However you will have a very difficult (if not impossible) time finding it anywhere else but at a Nissan dealer. I bought myself a couple of cases of the stuff. If you go to Auto Zone or Advance Auto or any other parts place, and ask them, they will tell you that if it specifies GL4, you can use GL5. That is not true. GL4 is safe for the "yellow metal" parts in your tranny (essentially the synchro rings). GL5 will cause them to wear out quickly. So go to your Nissan dealer and buy some GL4.
Another thing I have discovered is that if you take your car to your dealer for service or maintenance, they will not do what they are supposed to do. For instance, the little crushable copper washers on the engine oil drain plug should be changed with every oil change. There are also little aluminum washers on the tranny drain and fill plugs as well as the front diff and the transfer case. The FSM says to change them each time you remove and replace the plugs. But most dealers will not change them.
So my advice is to go to the dealer and buy a bunch of these parts a (copper and aluminum washers) and do your own maintenance.
Enjoy your X as I do mine.
Bruce S.