oil selection depends on alot of things. Service intervals, driving style, conditions, use of the vehicle, how often its driven ect ect. As a general starting point, almost all GTR's i know of including mine are serviced at 5000km intervals, oil and filter. If you use this as your daily drive, this may be a very expensive option for you so some people use a different oil with a longer service life and increase thier intervals to 7500kms. As for branding, stick with what is known. Mobil, Motul, Shell and virtually any of the big name brands are all good. I personally recommend a viscosity rating of 10w40 or when im racing i use Mobil1 0w40 supersyn gold, although its definately not what you would call cheap, its good insurance.
Like BWR has said, the oils thread is also a great source of info and experiences so happy hunting
Cheers, Allan