To be more on the afe side you can do 30k intervals. Infiniti recommends every 60k with 60k interval....

 

How much Trans Fluid is needed for a drain , fill on a 2003 G35? AUTO

 

^^^^
I think its 4-5 qts

 

Under no circumstances let anyone other than an Infiniti or Nissian dealer flush you're automatic transmission. It is critical that the correct transmission fluid be used. I had a new inexperienced Infiniti dealership flush my auto and they filled it with B&G fluid (B&G makes the flushing equipment) they ended up replacing my transmission because the B&G fluid damaged it.