I have always said (search the forums) that the first line of defense is proper machining/assembly of the half moon seal and of course proper venting is key to making it work as intended. An original engine, that does not have excessive wear can sometimes perform well if the half moon seal has never been disassembled. The reality of the situation is, only a small number of engines today meet that criteria. The half moon seal works correctly when the tolerance between the scroll and have moon seal are maintained at around .002 clearance. In the many years I've been rebuilding these engines, I've only had one that leaked at all after the rebuilt. This is a part of the build that I pay a lot of attention to, but I suspect maybe the machinist did not give this detail enough attention during the line bore.
In the event that venting does not solve the problem or a complete rebuild is "not in the cards", what are your alternatives? Either live with it or find something that works, short of an entire rebuild. I have experienced and also heard from many about the tablespoon to half cup of oil the engine pukes on shutdown. A few have been able to cure it with correctly plumbing the venting. I'd say conservatively more than half are unsuccessful with the various possible fixes, including the "JB Weld fix".
I think most of you know I have been working on an alternative to either of those. Reports are, when properly installed, my solution works... just saying there is another option.
Here's a recent thread on the subject on the MG Experience Forum.
Rear seal kit