Look for a Viton o-ring (brown) or silicon (red) in the same (or thicker) size as your presumably black o-ring. Viton is chemical and heat resistant, but hardish, while the silicon is softer but just as resistant.
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Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein