That picture of the cap suggests the bike is NOT an Atlas or Dommie.
If by any chance it is an Atlas or Dommie, the problem may not be the seal, but the leak may be coming from under the "L" shaped tab that is the oil tank top mount.
Vibration flexes the mounting tab, which in turn flexes the oil tank roof, and results in cracks under the tab. The leak seems to be coming from the cap, but is actually weeping under the mounting tab. I have the fix if anyone is in need.
I use cork in my oil cap.
Slick