Manual Transmission Fill Plug Stuck

[Sandrailstudent]

I am changing the MT gear oil (yes using GL4, and I agree with the risks of GL5). I got the drain plug out easily, but the fill plug is not responding to Liquid Wrench and a 1/4 inch impact driver rated to 155 ft-lb. Before doing anything detrimental, I figured that I would ask for some advice here. I will check the shop manual and make sure I'm trying to remove the correct bolt/plug. A photo is attached, if it will load from my phone. It is the only clean bolt, and I know everything is leaking like a sieve. I'm doing the output shaft seal during this.

I haven't had this plug out before, so I don't know what the PO did to make it this hard to remove.

If I can't get it out, has anyone filled the MT from inside the cab by removing the shift boot? I've seen some do it for other trucks on YouTube, but don't want to be the first to try it on this model. This would be a last resort, and I'll wait a couple days before doing anything irriversable.

Thanks,

Chris