Did a call at a home today. The owner said there was fuel leaking from the fuel lines near the burner. Obviously that's dangerous so I was sent right over.

Everything looked brand new. New fuel lines, new shutoff valves, new filter can etc.

The furnace was in the basement, the old one had originally been inside the house. I found a few very slow leaks, very very slow. A drop would form maybe every 5 minutes or so on the bottom of the shutoffs and around the gasket on the filter. I took apart the offending flare joints and it looks like the guy who did the install did a good job. No splits, cuts, burrs or any of that. When I put the flares back together I put a little pro-dope on them since I couldn't think of anything better to do besides tighten them down as best as I could. I changed the filter and gaskets and that seemed to take care of the leak on the filter.

Here are my questions:
Is there anything about kerosene that would make it leak more than #2 fuel oil? It seems a little thinner than #2 and I thought it may leak in colder conditions. Kinda the way PEX piping can leak a little when it's cold. The basement was chilly, maybe in the 50s. Or could it sneak its way through the threads if there wasn't any thread dope?

Was my thread dope idea ok? Some guys like the stuff and some guys don't. What's your take?

Thanks