Higher viscosity: "Locks" the diff down more, which basically means the diff won't slip as much. If you go into a corner and lift a wheel, the car is more likely to keep accelerating instead of sending all power to the free-spinning wheel. The heavier fluid may create a slight push as well.
Lower viscosity: The diff feels "loose", and the diff slips easily. This would help the car rotate in the corners better because the diffs are "freer" but if a wheel is lifted through a corner, you may loose momentum because all the power is being transferred to the free-spinning wheel.
I would run 5ooo wt. oil in the front at all times. This helps the car rotate well in the corner, but if you lift a wheel into a fast corner it doesn't "unload" very bad.
Run a slightly heavier oil (7ooo wt.) in the center. If the front tires are "balooning" down the straightaway, run a slightly heavier oil in the center.
In the rear, I would run 3ooo wt. oil. This will help the car rotate out of the corner. Sometimes, especially on a loose track, I'll move up to 5ooo or 7ooo wt. oil in the rear. This "locks" up the rear end and helps it slide into the corner (which can be helpful if you want to hit the throttle, slide into the corner, and straighten it out instead of rolling into the corner and powering out (which is where you would want the lighter oil)).
Hope this helps!