It's a 10.

When I couldn't get my front diff fill plug off, I welded a 10mm allen socket onto the plug. The plug was completely stripped from prior attempts. Once I welded the allen socket in place, the plug came out with very little effort. The heat from the welds really helps.

Good luck

It's critical that the Allen socket is perfectly clean before you attempt to remove the plug.
Also, when reinstalling do not over tighten.

Going to clean everything very well, heat the area around the plug, rap on it with a hammer for good measure, and give it a go. The plug looks to be in very good shape, not rounded or anything, so at least I have a good starting point.

That's the same experience we had when we shot this video. It wasn't so much the fact we now had a solid connection of the allen socket to the plug that made it easier to remove. It was the heat from the welding process that had the most positive affect. After we had that allen socket welded on, I was able to break the drain plug free with a 1 foot breaker bar with relative ease. On other rigs, it took me standing on a 2 foot breaker bar to break it free.

At one of my Timmy the Toolman parties, a guy wanted help getting his front diff drain plug off. I heated up around the plug once, tried with an 18" breaker bar and no go. I applied heat for a second time and tried again. It still didn't budge. I applied heat for a 3rd time and the plug finally broke free. It takes quite a bit of heat to get these stubborn plugs loose. The key is to not strip the plug. You keep on applying heat until you can get it off with a moderate amount of force. If you have to stand on a 2 foot breaker bar like I stupidly did one time and got away with without stripping the plug, you haven't applied enough heat yet.

Folks, I'll say it again. Heat is your friend when dealing with stuck fill and drain plugs.