Hi Joseph H,
when I unscrewed the plug, I noticed that the plug is actually steel, but an aluminum washer is used. Only breaking it loose gave me the impression it was all alu. So the washer is replaced and after a short trip no leaks.
Not so easy to get that fluid in there. Was able to pour about 1/2 quart out of the bottle, then used one of those plastic syringes that hold about 50 cc's....