As Colin says, you can get a Nismo oil cooler, which in itself does not invalidate the warranty, but as per any mod, if it causes a problem, then they wont pay out. It doesnt come with a thermostatic sandwich plate though, which is why they havent been able to get oil temps up. Over here you'd have to use a sandwich plate which allows the temps to rise before it diverts oil to the cooler. RJN would probably be the best people to speak to as they preped the FreeRACE cars.
Any mod in itself like an oil cooler, intake, exhaust, etc, will not invalidate the warranty, but if its proven to have caused the problem, they will deny work on the part of the car it relates to.