I've never ran LPG so I can't really comment on it.Not sure what gunk comes out of a forklift, but lpg burns very clean. Perhaps it's residue from additives some engines need for lubrication, the rv8 will happily run on neat lpg
For a V8 I don't thinks it that bad on fuel , well it's better than my dads l322 4.4 petrol ,fuel gauge moves more than the bloody rev counter.