Use 1 pixel, unless it's a special really thick line.
Here's something I do with making certain panel lines darker than others: I just make the panel lines, and once I'm done I add the blending/opacity changes. Then I duplicate that layer, select ONLY what I want to keep, invert my selection, and delete the rest.
That way I can adjust this other layer on top of what I already have to make the gear lines, or that one cowling line, or whatever I need, show up darker.