The filler plug on my NP208 was a beotch to get out too,when I attempted to remove it...someone evidently torqued it up REAL good when they filled it last before I got the truck...the fact the plug has very thin hex surface for the socket to grab dont help a bit either,and its brass so it rounds over quite easily...
I didn't have a socket that large that fit good,and my large adjustible wrench just threatened to round it off worse,so I looked thru all my tools trying to find something that would work,I ended up using a ainciecnt stilson wrench that was probably my dads that was made in the 20's,by using the tips of its jaws ,which look like a pipe wrench's jaws but are square and smooth and aren't spring loaded...I was about ready to just drill the dam plug and tap it for 1/4" pipe, if I failed to get it out and just put a pipe plug in it...it would have made filling it a lot harder though,with the smaller hole...