Racer Dave,
I put 150 mikuni jets in my two lines. Worked great for dampers.
Just happened to have two that I wasn't using. Suspect anything around that size would work.
I used Bardahl VBA straight up for a fluid. It's an oil we used years ago for racing two-strokes with 20:1 gas/oil mix.
If it digest any of this oil, no harm no foul. Just smoke. Then, just re-fill your gauge line and try again.
Hasn't happened yet!!! But have been very close.
Anyway, it makes a great gauge and very sensitive.