We run a clay track that dusts up a bit so we have medium grip and it is a bit broken up due to just running our state titles 2 weekends ago.
i'm running 5/7/1 in my diffs and i have more steering than i can handle, no joke.
The heavier the oils the less one wheel will unload and same front to rear as you go on the power.
In theory you should go heavier oil in slippery conditions to keep the power spread across the wheels and not just unload on the inside front wheel and lose steering.
In world rally cars they even run torsen diffs which keep power spread to which ever wheel has the most traction so you should try to get a similar effect with oiled diffs.
My set up works well and it is the same as the state champs set up too.