Is it unsafe to mix butane and regular lighter fluid together?
Butane has a boiling point of – 0.5°C and must be kept under pressure at normal temperatures in order to remain liquid. Lighter fluid used to refer to naphtha, which has a much higher boiling point. Wikipedia notes that the term is sometimes used to refer to butane, in which case your “regular lighter fluid” may simply be butane in a different package. You would be unable to recharge any kind of butane lighter with naphtha.
