All of the after market
oil pressure gauges that I have used have a 1/8 dia line. I don't know if it is ID or OD but if you go to a good auto parts store (Napa Auto Parts?) and tell them you need an 1/8"
oil line you should get what you need. Cut a small piece of the line off to compair it to. I used an electric sending unit ang gauge so I would not have
oil running into the interior of the car. Sometimes they leak.
Note: If you have enough slack line you can just cut off the end and reinstall it with a new ferrol (SP?)
Terry