I believe most elastomers will fit the task. Take the same hardness that works with natural rubber.
If the user may have already poured acid of sodium hydroxide in the pipe and then tries the choke pump, it is useful that the rubber resists the acid or base a bit. Search for a "compatibility list".
Natural rubber recovers its form fully and quickly. Many synthetic ones do it slowly, some incompletely. Keywords "creep", "setting".
Some synthetic rubbers can't be as soft as natural rubber. Thinner parts help to some extent.
You may want to check how durable each rubber is, especially to sunlight - but natural rubber isn't good under sunlight.
Some synthetic rubbers are extremely expensive. It's very possible that, once you find a dozen of synthetic ones that are up to the task, they all cost more than natural rubber. Begin with that selection maybe.