Its not about the oils.
Its says that the molded composite gears are smoother.
The composites have more give than steel because their not as hard. The steel gears being harder have a tendency to chatter because of the beveled gears. The viscosity of the diff oil helps to dampen the chatter. However when you use lighter oils (less viscosity) it results in less dampening.
Since the composites allow you to run smoother with lighter fluid, it is also able to react quicker. Given the size of these gears, I really don't see how the difference in weight would amount to any noticeable change in inertia between the two diffs that would have any great effect. However if one works more smoothly than the other, thats a change that would be noticeable.