[DeTomaso] Oil drain plug seal

Hi guys and ladies, Is it ok if I ask a question not about the POCA website or the BOD or copyrights etc.? (Sorry just kidding!) Anyway, I changed the engine oil and ZF lube this last weekend. I discovered that my oil drain plug had been seeping. It had a plastic washer on it so I changed to a copper one. I tightened it as tight as I dare but it still seeps a bit. Is there a torques spec for this somewhere? Any ideas on how to get a tight seal? Thanks, Dennis