Sorry if this has been mentioned in the last 687 posts, but I found the diff to be too tight. That is, my low-grip outdoor track needs a loose diff in the front. To tune the front end I started with the kit goopy oil, then went lighter and eventually went with no oil. Results were the same with each. That is, the diff was just as stiff, no matter which oil was used. It turns out (at least on my kit) the washers were too thick. All the stiffness was being caused by binding.
Probably everyone here is on high-grip surface. But if anyone is on low grip, I'd build the diff without any oil to make sure it is loose before proceeding with filling it with oil.