Does the G35x use synthetic 80w90(?) oil or just regular fluid? because a Nissan dealer is asking $66+tax for differential fluid change for regular oil and synthetic is $125+tax.

I don't think the X's came with full synthetic gear oil... however, its VERY easy to drain and fill, if you have a lift, or can go to a shop and do it there,,, I just changed mine a month ago, less than 30,000 miles, didn't look to bad,, Put in Valvoline 75w90 Full synthetic gear oil, This was on the REAR only,, have not done front diff. yet,, looks to be more difficult, but prolly not as dirty anyway,,, BTW,,, also change transfer case fluid recently,, that takes matic S

How hard was it to change the transfer case?

My car had both diffs. and the transfer case done at 32K mi, but I'm at 56k mi. now. I know I need to get the tranny fluid replaced.

the transfer case is very easy to do also,,,,, only got out a little over a quart,, but you'll def. have to get car in the air....