Not sure about that gearbox, which I think is Ford? The Mazda box in my Plus 4 has a large conventional bolt head drain plug with a metal washer, but when I took it off it was smeared with 'red hermatite'.
The quick fix would be to slacken it off as much as possible without losing the oil, then a shot of brake/clutch cleaner followed by a quick smear of sealant (I'm not a fan of red hermatite by the way - but there are other products available) - then tighten up. If that doesn't work then it may need the oil dropping. The big problem with drain plugs is that it's so difficult to get the threads clean. Some of the Ford sump plugs have an integral rubber seal and they are designed to be one use only - although so far I've got away with re-using!
