Oil & filter
I've got a GA16DE. I've used Bosch filters occasionally, but I don't think you can beat the stock Nissan filters.
Never run any oil in her crankcase except Castrol GTX. I use 20W-50 when it's hot here, which is about 9 months out of the year, ranging up to 113 in September a couple years ago; that same year, one night at ten o'clock it was 97!
During our "winters" (and often we go a whole year without a freeze)
Consumer Reports did an exhaustive oil test with a fleet of NYC cabs some years back, and after running them hard & breaking down all the engines, basically they concluded that as long as you get a good oil (Castrol, Quaker State, Pennzoil, Supeflo, Halvoline, etc) at the latest grade (currently SL, I think) in the weight that suits your climate, you should be fine. Just watch the level, change your filter, don't ever run dirty oil or cheap oil. If you feel better running an expensive synthetic (Mobil 1 or Syntec or whatever), fine--but this oil gets dirty like any other, so do NOT assume you can keep it in there longer.
But basically all oil additives (STP, Slick 50, whatever) are BAD news. Don't use 'em. A decent clean oil is the ticket.