That's why we got our fuel from a dealer. They made sure there was no undesirable weird stuff in the fuel (like antifreeze). They filtered all the bits of auto engines out of it, centrifuged the oil to removed all the unburnable contaminants. It was good consistent stuff.
Back in 1979 when I was working a Roaring Camp. We used to have one of the guys go around in a company truck and pick up waste oil at gas stations. We had a contract to get the waste oil from the motor pool down in Fort Ord near Monterey. We got a bunch of weird stuff. One batch had a couple hundred gallons of contaminated acetone in it. That got REAL exciting when a slug of that stuff went into the burner. To this day, I have never seen blue flames like that. It also dissolved a lot of the old oil in the locomotive's fuel tank causing the rivets on tank to begin leaking. This was back when we had such great gas that had real tetra-ethyl lead. Remember when regular gas was 95 octane? All that lead went through the burner.
One of the most paranoid issues at RC was......smoke. No smoke. At all. Ever. You aren't allowed to sand the engine out. That makes smoke (actually carbon dust, but...). So...every day I had to climb into the firebox with a wire brush and scrub off all this caked on gray dust, then with a flue brush on a long cable, punch out all the tubes to get rid of the gray dust.
I was informed later that the "gray dust" was mostly lead.
No wonder I'm not right anymore.....