The simple answer is run the rears (both cars) as loose as you can without them slipping then set the front a fraction tighter.
I NEVER tighten the diffs all the way down, always start loose and make small adjustments until I achieve the "no slip" characteristic. If you tighten them down you run the risk of brusing the plates and that will obviously limit long term smoothness.
Both cars like to run a fairly loose slipper too, so much so that setting the diff and slipper is a balancing act and after doing one you should check the other.
I normally set the diff with the slipper tight, then go back to the slipper and set that as I want finally going back to see if I can loosen the diffs a fraction without them slipping. So long as th eslipper goes frist it's all good
Using the Nuclear RC greases and setting with the above method gives me diffs that only need rebuilding a couple of times a year!
As for the grease getting dirty I'd clean it all and rebuild with fresh grease and see what if it happens again, most likely it's just all the new parts settling into a long and happy life together
Oh whilst I think about it are you greasing the gears? The cars like just a tiny bit on the teeth, as a rule of thumb I put about the same amount as you would on a diff ball on each pinion / idler gear and 2 or 3 spots on each crown / diff gear then turn the transmission over by hand (i.e. slowly) until it is evenly distributed across all teeth.
Doing that means not enough grease to attract the dirt but gears that will last a long long time (the ones in my cars are the originals from 3 years ago!!!)
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