with a BL system tour tires turning into pieplates is what happens and I dont think playing with the lube in the diffs will help( I might be wrong though), in on road applications I use solid foam tires to keep the tires from ballooning. I have thought about coating the tires with some sort of coating that will turn hard (maybe like a white glue) and this would help keep the tires rigid and not allow the expansion
I use 7k in the front, 10K in the center and 1k in the rear on my truggy. It definately unloads to the front a bit, but also provides stable driving and unloads to the rear on braking. On a rough track, lighter oil is usually in order(our track is bumpy). On a high traction, smooth track, heavier oil will improve acceleration. Anything above 20k or so is too stiff for my liking in the center on a truggy. On a buggy, I don't like anything over about 7k in the center. To each their own, though.
Mike's hitting it on the head here - it depends on your track. Our local track is kind of in between . . . not really rough, but not smooth either. In my XT8 I run 7k front, 25k center, and 3k in the rear. Seems to work well for me so far. Hope all this helps.
I would also say it depends on if you are using motor brakes. If you have a brake servo, you can run lighter weights. In my Muggy (bit heavier than a truggy), anything less than 30k and my brakes suffer (no stop so good).