I believe that's why silicone lube is used on the diff balls. It doesn't form the anti-friction layer between the balls and the rings like Moly would, but still provides lubrication from the balls and the spur gear - which under power is in constant contact with the balls. Using Moly would cause considerably more slipping due to how it forms a low friction layer when under pressure between two surfaces. Silicone moves out of the way for the most part.

Or at least that's how it was explained to me by some gray beard when I first got into RC.