I fitted a white nylon type seal. DON'T USE THEM!!!!!! when engine gets hot they go soft an plug loose only the fact I had safety wired it to the other drain plug stopped it failing out!!!
I now us dowty seals on Sump drain and Sump filter also on oil tank bango they are the steel washer with a neoprine lip seal inside the bore. Your can get them cheaply from a hydraulic supply firm or internet in the size you need an the seal holds back the oil so no need to over tighten things just tighten till you bit the steel washer. I still wire the Sump filter an drain together just to be sure.