This brings up an interesting and often frustrating issue for those of us who often work on our own machinery. Various rubber seals of this sort are often perished and replacements are frequently difficult or impossible to find. Rubber sheet we may come across might be perfectly suited for the job, or immediately decompose into black alien boogers on exposure to fuel, oil, or whatever. I've found that goretex gasket material is resistant to almost everything and makes a long lasting replacement for fuel, oil, and coolant cap washers and has many other uses as well.
regards,
Phil
