OK I got it a bit wrong - but jwalker did post some info on "boatmad" a while ago as follows, which I understand relates to the glassfibre tanks he's constructed and fitted to his diesel engined Humber:
Originally posted by jwalker
The resin used for these tanks is Neo Pentyl Glycol (NPG) resin. It has a very high number of cross-links in its structure. Consequently, the 'holes' in the structure are too small to allow most chemicals through. It is also heat resistant. I'm not sure of the molecule size for petrol and its additives in relation to the NPG 'holes'. There are two other resins which are even more chemical resistant, Bisphenol and HET acid. The bisphenol being better resistant to alkali.
Anyway, the big advantage of glassfibre over stainless steel is supposed to be the reduction in condensation.