I've used those plastic ones before, but I had one crack and leak, so no more.
I've used those aluminum ones with the molded O-ring of rubber inside the aluminum ... and just plain copper ones too. Then, there are some drain plugs with a groove intended for an O-ring but I can never find a new O-ring handy ... I have a box of them somewhere but if the O-ring is too fat, it'll get damaged.
I like the ones I mentioned that have the O-ring molded inside a aluminum ring that limits compression of the O-ring, and the copper ones are good as they can be reused after heating them really hot with propane, then quenching in water which softens the copper.
I've seen soft aluminum washers, but only used them on a Gold Wing MC oil drain, I guess they'd be OK. I don't know how I'd soften them for reuse though.
________________________________________ ____________________Problems huh? Then Read THE Book.
Signed: Natural Borne Neanderthal .