I think 15W50 is to thick if you're gonna use the car all year around.
It would take quite a while to warm up.
I would go with a fully synthetic 5W40 or 10W40
But as been said already, check with oilman
It also depends how tuned your car is.
This also affects the plugs you should choose.
I have Denso IK24s in my car, but I only use it in the summer. To dangerous on the ice
I would go with either iridium or platinum sparkies. But you could use regular copper ones. But you would have to change them more often though.
But changing all the fluids is a good idea. Just so you know it's been done. This is what I would change and the brands and stuff I used.
This is just what I use I haven't had much problems with my S15.
coolant (nothing fancy, regular cheapo)
engine oil (Motul 300V 5W40, running an oil-cooler though with thermostat)
gearbox oil (Motul Gear300)
diff oil (Motul 90PA)
brake oil (Motul RBF600)
spark plugs (Denso iridium IK24)
fuel filter (Nissan)
oil filter (Used Nissan first. Had to change because of the oil filter relocation kit I use. I now use a filter for a Rover)
air filter (Apexi
)
and look over various other parts like brake pads and stuff.