Taking the top cowl off to start my oil change this afternoon, I noticed blue staining on top of the engine. Traced it to the fuel pressure gauge line as it comes off the distribution block. It looked like it was leaking where the line attached to the 90 degree male AN flare, but the connection was tight. Found a tiny fracture in the line about a quarter inch from the flare. I am guessing from vibration fatigue or multiple cycles of removal & replacement or poor forming or faulty install. All of the lines to the injector look like stainless, but the pressure line looks like copper. Are these lines always copper? In my racecars I always used braided stainless hoses with AN fittings, for this very reason. I've also heard copper can "work harden" over time due to flex cycles, but I don't really know anything about that. Not sure what I'm asking, but just throwing it out on the web for any similar experiences or knowledge about aircraft fuel systems.