Thought I'd post some basic specs I've found (but don't remember where) for truggy diffs:

Starting Set Up
FR: 10K
C : 20K
R : 3K

Bumpy Conditions
FR: 7K
C : 10K
R : 2K

X-Loose Conditions
FR: 7K
C : 10K
R : 5K

What is the oil doing in the diff? For all vehicles:

Front
Thinner:
1. Increases steering into corners (off power)
2. If the oil is too thin then the steering may become inconsistent, especially losing forward traction (and steering) during acceleration out of corners

Thicker:
1. Increases stability into corners during braking
2. Increases steering on-power at corner exit

Center
Thinner:
1. Front wheels unload more during acceleration
2. Decreases on-power steering (reduces over-steer)
3. If a high power engine is used you could waste too much power and sometimes "cook" the oil in the center diff because it "overloads"

Thicker:
1. More all-wheel-drive effect
2. Better acceleration
3. Increases on-power steering (reduces under-steer)
4. Better suited on high-bite, smooth tracks
5. Car can be more "nervous" to drive especially if a high power engine is usedsmooth throttle increases may be necessary

Rear
Thinner:
1. Increases cornering traction
2. Increases steering into corners

Thicker:
1. Decreases rear traction while cornering
2. Reduces wheel spin

I am using a very high power motor in my truggy. That being said, I believe I have 30K in my center, 10K in the front and a mix (10K+5K) to approx 7K in the rear. My truggy is very stable at high speeds, puts the power down well but lacks somewhat in off-power steering.

Hope this info helps.