Well, since silicone expands so much when it gets warm, it pushes all the air out in the sealed diff, then when it cools it shrinks back down and sucks air back into the diff...

So you have to warm your diffs up to get the correct affect..

(Sorry I couldn't resist)

Speaking of,

If that theory is correct why does it not apply to shocks?

Only fill the shocks half way because they may explode or warp from the changing of pressure or because the silicone heats up and expands..