High compression engines like thinner oil... and yes, the lifters will tick if the oil doesn't get there-guess what's going on in it's absence....the manual calls for 5/30 and i run 10/30. I wouldn't go with anything thicker then that no matter what temp. 40 is too thick when hot and 20 way, way too thick when cold. go with what Nissan recommends and you should be good to 300k at least.
as to synth. I would put in mobil1 if i were you no matter how many miles. i just got my tk and am breaking it in to receive full synth in the future. I mixed in about 1pt of synth(10/30) and 3qts of regular mobil 10/30. next time it'll get a full qt. and so on. thus far no leaks but i did use a bit of oil (1/2pt. maybe) on my last trip-2300mi. No leaks thus far but if any develop i'll stick with dyno till i do the timing belt and thus many gaskets.... When you just start using synth oil make sure you change filter everytime or more often-depending on how often you do your changes, because synth oil loosens all the crap in the motor and guess where all that ends up....
I ran synth in my old 89 (RIP) and the oil was as clear as in the new bottle even after 100k mi. It ran like a champ at 175k and probably would've for a good long time....
BTW: if you do buy a new vehicle do not run synth in it during the break-in period!