I use 75w-90 Mobile 1 a la Billman. However, I change it every year. Why? Because I can and it is a great excuse to fart around in the garage with the dogs. It keeps the oil fresh and up to standards. I also change out the transmission fluid to boot. I use Amsoil for that.

 

@troubleshooter I thought there wasn't a need to have friction modifier in it? I've read it wont help but it also wont hurt.

 

It´s a Torsen differential, therefore it dont need Friction Modfier. But it dont hurt if the oil contains Friction Modifier.

https://torsen.com/faq/

From the Page:
How do a torsen Differential works?
It’s actually quite simple – by creating friction. Friction within the gearing is what resists wheel slip. When torque is applied to the differential, it generates thrust forces on the gears. The gears, in turn, push against the inside walls of the differential case, and that produces friction. In short, the more torque is applied, the more friction is generated, and the more resistance to wheel spin there is. See How it Works on this website.

Billmann has posted Pictures here, he found the "Ears" of the friction disks that hold them in Position at the side ot the Differential sheard of. They come out of the differential while changing the oil.
Sheard of by.... to much Friction?