Oil is not a solvent it's not going to wash you grease away. If it was properly greased before it'll remain greased with oil around it. I might give it a dab of grease when I checked the brakes, but really guys, that cork key way stopper is useless, sooner or later it'll leak and I'm sure the majority of trucks have oil in the hubs.
I had some things to add: You can cut a new paper gasket using paper from a magazine, someone here mentioned that, I tried it, it works. The cork create a gasket material is useless here I think the best best would probably be the rubber sheets pipe fitters use, but I haven't tried it.
You should clean the surfaces you apply silicone to, and after you apply it you should let it set up for about 5 minutes before putting your axle back on. This will prevent future leaks.
Thick impact sockets are your friend, chrome doesn't really cut it in this application. A 1/2" impact gun is also you friend, unless someone was watching them, I don't think anyone in the military would actually set the torque on those particular bolts.