Thicker oil is not the answer. The thicker the oil, the less the diff will unload. Meaning, the more of a locked diff you will have. The inside and outside wheels need to spin at different speeds to not be twitchy. If you have ever driven a real car with posi in the rear, then you know the feeling around a turn. The unloading of one of the tires make the car twitch. I run 7k up front and 5k in the rear. It is very smooth and still has great traction.

If you don't believe me, then put 100k in the front and find out.

If you put too light of diff oil in,the inside tire will loose traction when the weight transfers to the outside. This happens because the weight on the outside tire causes traction, but the open diff will unload and make the opposite tire to spin.