hey dude,
You will only need one if your considering thrashing the GTr at mallala, or you drive it hard in the hills on 30+degree days.
There are many oil coolers out there, ebay ones to jap ones.
Personally id buy nothing but the Jap ones, either HKS, Greddy etc.
Other wise you dont really need one the oil stays pretty cool in the GTR's until you start pushing hard all day or doing constant high speeds.
mike