Real rubber will. Pretty well all the carb rubbers are synthetic. Xylene is apparently one of the constituents.
My theory is that the rubber loses the light end solvents over time.
The good mixture is apparently Xylene and oil of wintergreen. Couldn't source wintergreen - methyl salicitate - in any quantity at an affordable price.
There used to be a guy - ex teacher - in ChCh who sold chemicals and equipment to schools and the public. Dead now I believe.
He was the only place to get a proper burette for measuring compression.
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