oil drain plugs are a different thread meant to seal on the gasket face.There are "self tapping " oil drain plugs available.Your engine drain plugs are a tappered thread meant to seal on the tiny faces of the threads.There is no gasket and these plugs should not be tightened all the way in as the taper on threads will seal before head is flush.Cranking these plugs all the way in often will cause the surrounding area to crack.A little rust on the thread boss can be cleaned with a small wire brush or as previously mentioned a gently used npt pipe tap of appropriate size ie 1/8 npt ,1/4 npt ,3/8 npt