I'm not smart enough to discuss theory, but in my experience going with thinner diff fluid in the center diff reduced wheelies on my 4s and 6s vehicles. For example, my Maxx wheelied terribly, even with a small throttle input. Based on what I read on Maxx forums, I swapped the 20M center diff fluid for 1M diff fluid. My Maxx no longer wheelies all the time, yet I can get it to wheelie if I mash the throttle fully in one quick movement. Changing the center diff fluid was like getting a different vehicle; one that is much more controllable.
My Kraton 6s and two Corally 6s vehicles never wheelied as much as the Maxx because the center diff fluid in my Kraton is 300K and the center diff fluid in my Corallys is 100K. The punch is turned down on all those vehicles, and I'm using throttle expo. The Corallys are faster than my Kraton, but they are more controllable.
I generally use twice the diff fluid weight in the front diff as I use in the rear diff. It works perfectly for me.