Rear End Fluid

I've got an 8-1/4 with a Dana Spicer Track-Lock in it, with Auburn gears.

The cover leaks a bit, so there's a nice little puddle under the car. I'm hoping to blow the rear end up soon so I can replace it with something stronger, so I'm not looking to fix the leak. I do want to fill the rear up, though... What kind of fluid should I use?