Thanks for the input. I really appreciate your help.
I think I've got it solved. The opening is the same diameter as a tap drill for M5 X 0.8 thread. So I tapped it for M5 thread. Tomorrow I'll go to my buddy's machine shop and drill a 2mm hole down the axis of an M5 machine screw. Cut the head off the screw, thread it in with Loctite Blue, and fit the oil line over the end. Problem solved. With about 10 or 12mm of thread penetration, that's never coming out by mistake.
Pictures to follow.