Going to change my differential fluid and 2 questions.
1 - The manual calls for 80w90 gl-5 gear oil if you're 4wd, 75w90 if you're 2wd. But it seems everybody just uses 75w90 regardless of if AWD or RWD. I'm AWD; is there any reason why I shouldn't use Amsoil severe gear 80w90? (i try to follow the manual for fluid specs since it makes me feel warm and fuzzy inside)

2 - I was thinking about changing fluid in the transfer case. I have ~100,000 miles on the car. I've never dealt with transfer cases before and was wondering if they operate on the same theory as automatic transmission fluid changes - i.e., if you havn't been routinely changing the fluid, don't touch it now otherwise you could cause unintended problems. Or if they're safe to dump and re-fill

 

1. I've used 80w90 for multiple diff. services - have never had an issue.
2. Not that I can see. My thought here is the T-case doesn't see the same use the transmission does. Drain and refill with fresh fluid would be my rec.

 

Z1 has all of the kits needed to do the diffs, transfer case and trans. I would look there and you should find all of the info you are looking for and the kits are reasonably priced with new washers etc