Nissan recommend 7.5W - 30 in their books.
+ the R32 being engineered in the late 80's to early mid 90's, it would most probably be a Mineral Based engine oil.
I have used:
- Martini Racing 10W - 40, Full Synthetic = Very nice!
- Valvoline 20W - 50, Semi Synthetic = Bit too "thick" and car ran sluggish.
- Shell 15W - 50, Semi Synthetic = Was ok for the first 1000 kms then turn to shit.
- Mobil Synthetic S 10W - 40 (2000 X 2) Semi Synthetic = Best so far from what I have used and just before an oil change the car still runs well, once changed = Very smooth.
My mechanic has recommended Mobil Synthetic S and the car is running fine.
I think the Mobil has the closest viscosity to what the R32 GTR should run, but everyone has their own personal preference and recommendations.
Different cars will have different recommendations + if you car is modified (Mild to Heavy) and having internal mods done, you'd run a Full Synthetic as the engine will have different tolerances to stock and lightly modified cars.