Oil pressure line

[davepl]

For what it's worth, I think copper is probably safe, but it does work harden. If you have it in a an area where it'll get repeatedly bent or flexed, it'll fail eventually. Imagine if you had a oil pressure gauge on your hood, for example, and every time you opened the hood it'd flex it.

Mild steel is much more resistant to work hardening than copper, which is maybe why the factory went that way.