There are two transmission coolers. One is an external cooler located in front of the radiator. The second cooler is located inside the bottom of the radiator. This is the cooler that is prone to leaking and allowing engine coolant to enter the transmission. You can eliminate the possibility of transmission fluid contamination by doing any of the following:
1. Replace stock radiator
2. Install a second transmission cooler in place of the cooler in the radiator
3. Just bypass the transmission cooler in the radiator.
Many people choose option 2 and I would recommend this if you do any towing. Many people, myself included, also do option 3. I would strongly suggest you do option 3 at a minimum and you will have a greater peace of mind. Oh and welcome to the forum!
If you need any more details check out this thread.
http://thenissanpath.com/viewtopic.php? ... ght=bypass