I never had a problem with the stuff that comes with the kits. At least so far, so good.

If you want a colored or semi clear line, go to a local hobby shop (model airplanes) and ask for GAS line, not for GLOW fuel, but for GAS. They will have a selection of colors/sizes.

Or go with good ol' black.

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If it ain't broke, and you mess with it long enough, it will be.