we do legally have to run filters inside the LPG lockoff defore the converter. But the black sludge, called rough ends, are not dirt in the fuel but a by product of converting the liquid into gas, just a nother result of poor refining in this country. As for pricing here, LPG is currently averaging 50c au per litre which works out to be $2.25 au per gallon. Which sucks as the LPG sold here in aus comes from Bass straight which is local and our LPG in final refined form is sold to China for 3cents au per litre. we pay about 16 times that for not fully refined LPG. There are only 2 places in Melbourne that I know of that sell PURE propane to motorists, they are gas distributing depots.

That system are you using, looks like a complex converter with a 'spud' mixer.