I torque mine to 51 ft-lbs. I only recall 1 torque value in my FSM. I don't think there is much to consider for yours, I believe you have a stick or breather hole to fill, so hard to mistake that for a transmission fill spot.
Maybe confirm by smell that where you fill (hose or stick) indeed has a gear oil smell.

 

Nobody has any torque recommendations? Someone other than @mjohnson has to have drained their differential in their Gen 3. Should I torque to 51 ft-lb or just “snug”?

 

Could also have said my washer is steel. If your FSM says to use 51 for Cu, then the threads can take that torque. If you want to be extra careful, go 40-45.
Is the drain bolt a torx T-70? Also what mine is, beefy bolt.

 

Sorry to jump in when I haven't done this myself but I think the reason why a low torque is used with aluminum is so that the gasket doesn't get split, so unless there's some reason why a steel one would need a low torque I would use 51. Presumably there's no risk of stripping the aluminum threads at this torque so it should be safe.

 

Good point. I just wished that Subaru had revised the FSM when they eliminated both aluminum and copper gaskets and switched to steel. Unlike the oil drain plug crush washer, this “gasket” is solid steel. Kinda like a thicker washer.

When I received it it didn’t look like aluminum or copper so I did the magnet test and it’s absolutely steel. It’s actually almost black in color. I’m assuming it’s some type of rust inhibitor.

 

Ditto re not having done this. But if I can bring other experience to bear, if the copper washer is solid, that would explain the much higher torque. If the "aluminum" washer is really steel, then I might go with 32 since 30.8 is the engine drain pan plug spec with steel washer? The main factor is sufficient "crush" without tearing/deforming the washer/gasket. A magnet would tell you for sure if what you have is steel.

 

I doubt you will get a seal with a flat steel washer unless you use some form of sealant as well.

Aluminium and copper washers seal because they “deform” to the shape of the material they are sealing to.

If it was my vehicle I would definitely be using some type of sealant. My sealant of choice is non-hardening aviation cement. It used to be a Permatex product but I believe it is now made by Loctite.

Seagrass

 

This is an OEM gasket that was purchased from my local Subaru dealer. How can I verify that this is the correct one?

When I realized that it was steel I had the same concern. It’s not like the oil drain plug steel crush washer since this is solid. Yes, copper and aluminum is softer so it will “form” as it’s torqued.

I wonder if SOA could assist?

 

I wonder if @Grum knows?

 

I've been using the black steel gasket since it was introduced (faced the same dilemma), and have been following the 51 ft-lb spec, although actually using somewhat less as the spec is "dry" but that's almost impossible given the nature of the gear oil.

 

For what it's worth (@FlatFourAWD ) I've done 3 diff fluid changes and reused the factory steel gaskets every time. No sealant ever used, wiped the gasket before reinstall. No fluid leaks (garage floor clean). The seal is made by the threads being tight as heck and the flat surfaces (diff/undamaged washer/bolt) well mated.

SOA will tell you to use what they sold you or go to a shop.

 

Thanks everyone! Now I’ve got some work to do!