I dont think an oil cooler will void the warranty as long it doesnt cause problems to the car. If Nissan is installing an Oil Cooler on the Nismo, which is the same car, we can do it too. Plus any aftermarket part is protected by the Magnuson-Moss Law Act. THe problem is that there's a lot of people don't know about this law, and a lot of dealers that either don't know or want to ignore it.
Magnuson-Moss Warranty Act - Wikipedia, the free encyclopedia
This is what I use against dealer when they try their BS warranty with me.