Oil for hardness tester?

[A.McPherson]

Hey folks, I finally got around to fixing a harness tester I've had for a while, but the dash pot is empty. What kind of oil do y'all use?

I've got 10, 20, and 30 weight, non-detergent oil, that I use to lube my lathe. Will any of those work?

Thanks!

 

[DeadboxHero]