Ah, back to chemistry class days...
Refining is basically a fractional distillation process where the raw mix is boiled, then because each component condenses at a different temperature you set your equipment up so you capture each fraction (component) as it cools and condenses. I presume with the equipment in common use there was a fair amount of cross-contamination of the various fractions. So a second distillation would help purify each fraction. Then you mix the purified fractions to your secret recipe (and add urbs and ices) to get your snake... er hot rod oil.