You can make it a little more efficient. Look into an exhaust. Karmann has a nice sounding mangnaflow? (I can't remember the brand) and I know the VG motors response very well to a cat back exhaust. Well have to ask him if he saw any MPG improvements from his exhaust.
You can replace the O2's if they are original (just fronts, rears don't affect milage) it might help.
And yeah, having drive train fluids served can marginally help too. Checking universal joints along the drive shaft can also be a worth while venture, if they are worn due to corrosion, for example, that can cause excessive strain on the engine.
But by far the best mpg mod for the pathfinder is manual front hubs. You lose the option of 4WD on the move. But I've had mine for a few years with no issues. I had about a 2 mpg increase before the exhaust issues.
So if I were you, I'd look into manual hubs, they definitely help.
-Kyle