Start by giving it and the surrounding area a good clean with a stiff wire brush. Next, a good dose of ‘Plus Gas’ and let it soak a bit . If the internal hex has rounded, one option is to use a torx bit that is slightly too big and tap that into the internal hex, then try again. The other option is a decent pair of ‘ Mole’ grips on the outside of the plug . It should not be that tight, but corrosion and perhaps an unknown history has left it so. As it is the fill plug and the level plug, it would be nice to be able to remove it . Filling from the drain plug would qualify as a scene from “mission Impossible” ’