I think Bob's talking about an additive to coolant, not fuel; I'm not familiar with DCA.
Jack,
There CAN be water traped in fuel/WVO/biodiesel/hydraulic fluid/whatever you burn. Bacteria can grow at the oil/water interface. Some have the ability to digest oil and form more water as a metabolic byproduct which gives more bugs a place to grow, which . . . you get the idea where the horror stories come from about diesel tanks growing "algae." Others may be sulfur reducers that can feed on sulfur in the fuel, forming stinky hydrogen sulfide in the process. Though they're sometimes referred to as algae, I've always read that they're bacteria, but the distinction between the two has become somewhat cloudy since I was in college. Some of my favorite blue-green algae are now classified as photosynthetic bacteria. 'Course the same kind of people now say Pluto isn't a planet, so who gives a hang?
Fuel or oil or whatever you're burning by itself won't normally be corrosive, however if there is any water in it, the water will eventually settle in crevices and under the right conditions it can promote crevice corrosion or can become acidic and THAT can cause problems. I suspect the sulfur level in the fuel is a factor there. If you're sure there's no water in your fuel, such as after a fuel filter/water separator, copper/brass/whatever should be OK to use. However anywhere there could be any water in your fuel/oil/whatever, it's not a great idea to use copper.
Yes, brass and bronze fittings are common on the exterior of old engines. Even marine engines used to have a lot of brass/bronze. I don't see a problem if the brass is kept out of contact with water. In a working situation everything would have been covered with a thick coat of paint, so if the duffers want to strip the paint and shine up the brass, more power to them!
As for copper vent on a steel tank, it's OK to use as long as you go through a bronze fitting between the two. At the very least, really goop up the threads with anti-seize grease. All steel would be better, though. Use whatever you have handy but keep an eye on it. If you see a problem developing, then change the fitting or vent line.
Quinn