According to the manuals, the military steering knuckles use grease to fill not gear oil. Civilian applications use gear oil. Some people on both sides of that fence make a mixture of about 50% of each gear oil and grease and put that in...it has the nickname of "knuckle pudding". This keeps knuckles that tend to leak gear oil from leaking too much but lets better cold temp lube happen than with grease.
The only way I know of the really clean it out is to open it up...that said, if one got the steering knuckle good and warm on a warm summer day from a nice drive...or maybe like a magnet mount block heater?...then open the drain plug and let it flow.
As far as the steering, I could just get the top hex filler port open with a crescent wrench from the front...tight spot...as far as emptying it, I think one would have to take it off and turn it over or somehow suck the stuff out...
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!