think of this like a 2wd jeep at the beech. if you drive on the sand , you have to be careful, of you will dig out under a wheel ( that wheel is stuck in the sand, when it spins, it does nothing in moving the car) . even though the other wheel is touching the ground, it has no power because of tthe differential, putting all of that power to the usless, spinning wheel.
now, lets say you put in a thick, stiff oil. the differentials action is slowed/limited. so both of the rear wheels will then have power, and be able to get allong through the sand.
that is similar to 2wd buggies. rougher conditions may require thicker oil. while smoother conditions use thinner oil.