Didn't take long to jack this dudes thread.
When you are on power the diff sends the power more power to the front wheels as they begin to loose traction. This scrubs alot of speed. By adding thicker oil you are in a sense locking up the diff so it continues to send power to the rear. The thicker the fluid the more pronounced it is. Go to extreme and you will be spinning out whenever you jump on the throttle. This is a good way to control forward traction. I agree with the others that you are best to start light and work up until you see a loss in performance. Then bacl it off a bit. Its up to your comfort zone to figure out.
If you are doing some wacky project and want EQUAL power to front and rear. Just lock up the diff with some epoxy.