go by the book!! I use 10w40 semi synthetic castrol magnatec.
Cheap oil (Nissan genuine one) is better if you change at regular intervals e.g. every 6 months or 9000 miles. Which ever comes first. GTX degrades slower but causes an acid build up in the process so it should be changed every year regardless. The magnatec stuff claims to stick to the moving parts in the engine better. So when the engine is off the parts will be ready to go and there will be less wear and tear.