How would air in the line give a false reading? If you put 20 pounds of oil pressure onto the pocket of air, it would compress to 20 pounds.
Re the line, while I can't speak as to the correct size for the Chev (but have usually used 1/8 tube with compression fittings), I would advise that you usually add a pigtail to the line at the engine to absorb the difference in movement from the engine to the cab. Otherwise, the line can, over time, become brittle at the point where it absorbs most of the flex, and break.
The actual thread for a 1/8 compression fitting is 5/18 x 24, the same as used on a 5/16 UNF nut or bolt. If that is the thread on your gauge, then that will be the tube size used.
Last edited by rob love; 13-07-19 at 17:17.
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