I've been waiting for a picture to make sure I understood what your were having trouble with. Luckily David came through! And to David, I finally had to replace my o-ring on the overdrive after 4 years and 5 oil changes. If you aren't anal about oil changes like I am I think it will last decades!
That is a freeze glug, or at least what we've all come to call a freeze plug, as it has nothing to do with freezing really. But, back to the point, it is a "welch expansion plug" in British terminology. If it is leaking and you plan to drain the fluid anyway, I would recommend removing it and replacing with a new plug. I am sure one of the big 3 will have one, and if not, then measure the size and you should be able to find a suitable replacement in the auto-parts supplier market.
The plug is cupped, so that as you install it it with the convex surface downward, is slips into the hole. Then you wack it with a hammer in the center of the convex, and it oil cans into a concave surface facing down, and that locks it in place. Normally they never leak...but since yours now has a history, I would recommend a very small amount of sealant around the plug as you install the new one, just in case the hole has been scarred or otherwise damaged in the past.
Just adding sealant to the old plug may work for a month or 2, but it can't seal in the cracks once oil is saturated through them.