Re: what is this not quite an o ring?
I believe I know the gasket you speak of. You will not likely find an off the shelf replacement for it. Here's three ideas that may help out.
1 - Take the broken gasket to several auto parts stores and see if they have anything similar, you never know what you'll find.
2 - purchase petroleum resistant gasket material and make your own gasket.
3 - Find a hydraulic repair shop in your area and take the gasket to them. They may have a similar material that they can make a custom gasket from. Believe it or not, this particular idea saved an antique butlers tray from the heap yard when a client needed new rubber wheels for it. The point is, thinking outside the box will be the order of the day.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!