Harley uses neoprene or butal. Viton is a more resistant to tearing.

But good grief get a bag and replace them every time. In bulk they are only about 10¢ to 15¢.

 

If you are having the same problem with the O-ring tearing as I am then nobody else's O-ring will fix the problem. It's not a matter of the O-ring wearing out or being installed wrong. Perhaps others just don't understand that there isn't sufficient room for the O-ring when installing the drain plug. I could run through that bag of 100 O-rings in an afternoon. It's a defect in the way the plug or the drain hole was made. Oil on the ring won't help (tried that). Wrapping it tightly with teflon works with an already damaged O-ring. That just means there is something wrong with the drain hole or the plug. I've tried everything including waving a dead chicken over it! Next time mine's out I will try to ease the edge of the drain hole with a countersink bit. Just enough to prevent the O-ring from bulging out. It bulges out because there isn't sufficient room for it to live in the drain hole when the plug is tightened. If it gets under the head of the drain plug it won't allow the plug to be tightened sufficiently.

This is just a typical H-D type of problem. Annoying!