As I understand it automotive rubber, nitrile butyl rubber, which is resistant to petrol is also very hard. It has to have a plasticiser added to make it supple enough to use but ethanol added to E5/E10 fuel cheletes the plasticiser. Consequently the rubber becomes brittle and cracks/fails. It is important, therefore, not to have rubber at all and use silicone but many older cars have NBR components.
This is as I understand it but it may be that a Talkmogger will correct me if I am wrong!
