there is an oily resideu propane is a bijproduct of oil as it hardly needs heat to get extracted from crude
propane used to be and sumtimes still is being torched of at the drillinhrigs it was considered a wasteproduct and the transport of it not viable
it doesnt realy get distilled at the refinerie so oily vapor that sticks to it from the crude is compressed along with it in the tanks
on most impco systems the caspart of the twin fuel system is above the carburator and the gas passes the carb if you run that twin fuel car on propane for longer periods of time it can clog up the gasoline stuff with a brown/lack tar like substance making running on gasoline nearly impossible until you clean the carburator out with carbcleaner the ones that are mounted below the car dont have so mush problems
you sumtines need to open up and clean some parts of the vaporizer too but then again you need to do that on a gasoline engine's carburator too after an x amount of miles on it , it still is a verry nice fuel to run on
all gas has some residue if you ever get to feel around the inside of a used natural gasline you get the same smudgy finger from the residu , after they replaced my gasmeter i got to see that myself
