Depends how tight you want it. If you want it really tight, like relatively hard to turn by hand, then you wont find an oil that will do that. 100k will give you good torque distribution, but is still loose enough to keep the rear end in check. Start from there and work up. If 500k still isn't enough for you, stick with the spool.
In the past, I've used actual car diff oil. SAE 90. When its confined to such a small space, it loses alot of its fluidity, and creates a tighter acting diff. It doesn't give the diff a locked/spool affect, it just makes it tighter under high diff speeds. Adding a little friction modifier, has yielded some good results too. This is only a personal preference though. I prefer this, because it makes the truck a little easier to put into a slide. If thats what you want...
