I can't really get into the warranty stuff, but it is considered an "Competition Part" thru Nissan Motorsports Competition parts. I would recommend talking to your local dealer about your concerns with oil temperatures, and have them document it into their service history. Let them know that high oil temperatures can cause engine damage and that this oil cooler is preventative maintenance to prolong the engine's life. I would not recommend using the oil cooler in conditions where freezing or cold temperatures may be present. If used for racing only, it may be used in colder temperatures but for a street vehicle, it may be too cold. If cold temperatures are a concern, you could try using the oil cooler during the summer... and stock for winter.
There are many things regarding the oil temperature concerns going up the chain to Nissan... They may or may not void warranty.
Magnuson-Moss Warranty Act - Wikipedia, the free encyclopedia
A Businessperson's Guide to Federal Warranty Law