[BillV]

+1 on that. It could be a leaking valve cover or many other places in between. No offense but you may be so fixated on the plug that you're ignoring the real source. Here's an old but relevant post. If the leak really is at the plug, a suitably sized Dowty or Stat-O-Seal sealing washer might cure the problem.

 

[BillV]

If it works, it works. But all that suggests that either the sealing surface on the crankcase or the plug is damaged in a way that the copper washer won't seal.

 

[BillV]

There's a thread here where I switched my drain plug to an aluminum plug with magnet and a Stat-O-Seal. I see about one drop leaking out every 3-6 months. I've since located a fairly high end NAS seal intended for aircraft usage but have yet to use it.