My tank rubbers - archaeologists will puzzle millennia hence over a cache of fossilised tank rubbers found beneath "what may have been a primitive workbench", because I've lost more than I've worn out - have always needed to be fitted initially with a lubricant. My personal preference is talcum powder, which has the advantages of 1) not altering the properties of the rubber, and 2) reducing the stock of "men's grooming products" that lazy relatives have sent as Christmas presents. It works well on tyres too, allowing them 1) to be slipped easily over the rim and 2) not gumming themselves permanently to said rim, as will be discovered when next one wants to take the tyre off. This cannot be said for washing-up liquid.
It might be time to try tipping the aftershave (bearded men don't use it) into the fuel tank: might do less harm than modern "petrol". Hell, it's 5% ethanol...
Morecambe and Wise joke:
Ernie to Eric: can you think of a sentence with "judicious" in it?
Eric: "The hands that judicious can be soft as your face, with Mild, Green Fairy Liquid".
There's no answer to that.
As always, Trevor is correct. I think the correct terminology should be elasticity. Then again, surely some flexibility is needed to do the desired job? I don't think perished rubber that is as hard as a rock will do the job. I have a rubber ear/nasal syringe that is about 100 years old. It has the flexibility of a lump of coal!