Make the moron that put it on so tight get it off! If you have to have a breaker bar to get it off it was put on with too much torque! Easiest way to get off the OEM hex head is a 24 or 25 mm, do not recall which it is, weld on another nut or air chisel. I cannot tell you how many OEM ones I have had to PERSUADE off.

Replace the OEM fill and drain plugs with a 10mm sunken hex head like in the picture above, get the 10mm hex socket, and be done with it. Only difference from diffs to transmissions and t cases is the washer, either steel or aluminum, depending on application.

We have both in stock on the website and the description tells what you need for what.