if you are talking about the haldex oil that gets replaced every 20,000 miles with filter, than (you're not gonna belive this) it is filled through the drain plug! the new oil comes in a tube to fit a chalking gun. once drained the fluid is is injected upward into unit then quickly capped by the drain plug. the tube has a little extra volume to account for minor loss. other diff gear cases are filled by a level hole in the side of the unit to be filled, but these are considered lifetime fills.