4L30E Drain Plugs

When I first pulled the drain plugs on my auto tranny the washers were a strange sort of composite metal/rubber. Like a thin metal washer with 'fingers' on the inner diameter, supporting a soft rubber ring. The reason for this seemed to be because there wasn't any machined sealing surface on the pan for a conventional washer, just raw casting. It looks like the rubber ring is meant to get mashed into the plug thread.

My dealer wanted about $25 per plug, so I re-used the old washers, needless to say they leaked.

The next time I used aluminium washers, but at the last minute I put some thread sealer on the last couple of threads. I don't really want to do this in future but I don't want a leaky tranny.

Questions are:-

Are all 4L30E's like this with no machined surface for the plug sealing washers, or is this another special gift from Isuzu for export vehicles?

I've seen posts saying to use copper washers. How effective is this on the un-machined casting (if that's what you have)? I guess it would be softer than aluminium. What about fibre, I've seen white or black but don't know the difference? Any other methods used?

Thanks,

Andrew.