Questions about oil cooler and differential gear oil

[Rhett]

Hi!

I have the 2009 370z with touring and SP. My questions are as follows;

1. When is the rear differential gear oil changed? By maintenance manual, Nissan recommends to change it around 30k miles. But, some people say that it should be changed within 2~3k miles. Which one is correct?

2. I know that the high temp of engine oil has been a hot issue only for the 370z for severe driving conditions, especially during Summer. Do I have to install oil cooler on my z? If do, which one is the best? I know that this question is too silly. Will you recommend the cheaper and good one? ^^ Stillen is too expensive. Also, will you let me know auto shop to install the oil cooler system around Seattle?

Thanks

Seongkeol in Seattle