[JJ'65]

Well...

Autometer is a reputable company selling quality stuff, and they probably wouldn't like to be in court about product liability, so there's probably a good reason why they include a nylon oil pressure line. Nylon is pretty tough stuff, but it melts at a lower temperature than copper.

Copper doesn't have much resistance to metal fatigue, so with some "working" or vibration it will crack or break too. If i was doin' it, I'd use the stuff that came with the gauge and protect it good from heat and sharp/rough edges that might damage it (same goes for copper). Instructions probably mention that.

Just my $0.02