From an airplane builder's point of view, we used copper or stainless steel tubing. The thing is, you should use a strain relief between the engine and firewall. What is a strain relief? Make a small one or two ring circle like a pigs tail. Your engine is always moving, your firewall isn't. The pigs tail allows flexibility. Copper can split in this region. Friend of mine almost lost his airplane from a fuel primer line burst, it had finally yielded from flexing. The engine had a backfire when starting, caught fire. Luckily, he got the engine started and the fire was distinguished. Found 3 bad copper lines. I have never had an oil line (plastic) burst, but everything at some point, can fail. Just do it as good as you can.
