One thing to remember about propane. The colder it gets, the heavier it gets. So when the temp starts down into the single digits there is hardly any vapor. And the oil thing in propane, yes there is byproduct waste oil in propane that gets there through the manufacturing process. Having worked with propane fuel systems for over 25 years I have only seen a couple dozen systems that have had an issue with contaminates in the system that would cause a problem. I have inspected LP bottles that are over twenty years old and not found large quantities of contaminates in the bottom of the tanks. I have changed valves in -30 weather that allowed me to pull the valve out without having to empty the tank. You could look down into the tank and see the liquid.
Hank Hill, is that you??!!??
