BVM has this posted on their web site under the fuel fittings.

NOTE - Aluminum Tank Cap Fittings purchased during 2003 have rubber stoppers that can deteriorate due to additives in the gasoline from some states such as California and Indiana.

The black rubber stoppers purchased from BVM post January '04 are ok. Of course, BVM will replace any problem stoppers at no charge.

NOTE - There is no problem with any black stoppers using Jet-A or Kerosene fuels.

Maybe there is some additive in the fuel you are using??