I've always used AMSOIL 75w110 (severe gear) in my stock diffs. I'm switching to a 28 spline 8.8 diff shortly and will be using AMSOIL 75w140 (severe gear).

 

Last time I changed the diff fluid I used some LE1605 I think. It came highly recommended, but seems like it's not as easy to find anymore. If I remember correctly the diff only takes 1 quart right? Next time around I might just use some amsoil 75w110.

 

The diff takes just under 1 quart, yes.

 

I have always used the standard Honda oil for the transmission and I like Mobile 1 90w in the diff. I ran this in my stock diff for all those years on the track. My transmission syncro's are still good and I never had a diff problem. The current OS Giken diff calls for special, very heavy, very expensive oil.

 

I used Mobil 1 75w90 when I did the 60k mile maint on my AP1 a few months ago.

 

Seems like it's performing normally.

M1 looks to be popular.

 

I also use Amsoil 75w110 severe gear. Amsoil makes quality products

 

Also, make sure you have a 24mm socket and a 23mm box end wrench before attempting to change the diff fluid...

 

And ALWAYS loosen the fill bolt first. You'll be stuck if you drain the diff fluid and can't get the fill bolt off...

 

^ This.