Depends on personal preference, but I have 5k in the rear, and 8k in front. I dont know much about that side of things really, but all I do know is you generally have heavier oil in the front, to aid steering (sling the truck around corners better).... or something like that I think.
I'm using 5k in the front and 1k in the rear for bashing. That combo seems to do pretty well, good combo for strg action and traction. What works best for you will partly depend on surface/traction you're running on.