The green o ring is made from fluorine rubber. It is misnamed by the engine folks, but that’s what it is. It is commonly used in aircraft and food processing machinery, and withstands petroleum and stresses well. These engines were initially shipped with two piston rings early on, which proved to work too well, no oil getting between them. They’re now shipped with one, and reports are of much improved life, as long as even a little marvel mystery or other suitable oil is used.