One of the recommendations by my surveyor was that I replace my diesel
sight tubes that were vinyl tubing with polycarbonate. I did and no
problems several years later. As far as fire, they should be isolated
with valves that are always closed unless you are actually checking
fuel levels. Polycarbonate tubing is what's used extensively in the
petrochemical industry for sight tubes for diesel, gasoline, etc. You
can find it online, or at service station supply places.