yes. think of your diffs like you do your shocks. When your shocks are not filled completely and have air in them, they are easier to compress than a shock completely filled with oil because there is less resistance when air is in the mixture. so when your diffs have air and oil in them, they make bubbles and don't give you the desired "diff action" that you intended it to have. when it's completely full, there isn't any air (or hardly any at all) and the diff wil operate as intended.
