I see people recommending the redline 75w90 NS in the trans, but 75w90 in the rear LS diff.

I've searched, and that one has the friction modifiers for LS diffs, but from looking at the redline site, that's for the clutch in a clutch-type LS diff. I thought I had a viscous coupling in the read LS diff, so I would think the lower friction of the NS oil would be preferred.

The viscous coupling is actually up front. It's where the transmission and front differential are located. The rear differential is just the rear differential.

The transfer gears and center differential are located in the transmission housing rear.

Most people put the NS in the transmission to get easy shifting. Most people don't put the NS in the rear differential.

The viscous coupling has a special fluid in it and you don't change that fluid.

You can read up on this in "Mechanism and Function" Service Manual number 8.

The viscous coupling is actually up front. It's where the transmission and front differential are located. The rear differential is just the rear differential.

The transfer gears and center differential are located in the transmission housing rear.

Most people put the NS in the transmission to get easy shifting. Most people don't put the NS in the rear differential.

The viscous coupling has a special fluid in it and you don't change that fluid.

You can read up on this in "Mechanism and Function" Service Manual number 8.
That's how I understood it for the MT & center diff. My rear diff is also LS, and I'm pretty sure also viscous coupling, not clutch type. So I'd expect to just use the same gear oil there as in the trans, 75w90 ns. But when I search that's not what people say to do.

My rear diff is also LS, and I'm pretty sure also viscous coupling, not clutch type.

You are correct. The rear LSD is a sealed unit and uses its own silicone-based fluid. The rear diff fluid does not touch the internals of the LSD. It functions very much like the viscous coupling that serves as a center differential up in the transmission.

Back to your question... I would imagaine that technically, you are pretty safe using either one. They are both GL-5 oils capable of handling the shear incured by a hypoid gearset. However, Dave Granquist (Redline Oil's tech guru) always recommends 75W90 for all rear differentials, even those without an LSD.

I don't know why, but on the few occasions I have spoken with him on the phone, this is always his recommendation. Basically, he says to use good ol' 75W90 unless you have a special application that requires something different or are encountering some issues with the friction modifiers.

However, Dave Granquist (Redline Oil's tech guru) always recommends 75W90 for all rear differentials, even those without an LSD.
Good enough for me then. Thanks!

Holy cow Batman! You guys are right there's a viscous coupling in one of the 2 types of rear differential, in mechanism and function for my 02 impreza.

So I've got two of them. I just checked the Window sticker (yes I saved it) and its LSD so it has the viscous coupling.

"The space between the differential case and the left side gear is filled with a mixture of high viscosity silicone oil and air and hermetically sealed with X-rings." (X ring is repeated as no. 10 on the diagram)"