I agree with the WD40 suggestion, or white spirit, DERV, or Paraffin as a wash (not in the Ultrasonic machine) before rinsing in detergent water in the Ultrasonic bath. Actually, my favourite solution from the kitchen cupboard is the clear window cleaning spray liquid. What you are doing is lowering the surface tension that holds particles onto the metal surface, so many fluids do that. The common term is simply "wetting". A toothbrush also works well for scrubbing inside bores.
Don't use hydrocarbon fluids in the Ultrasonic machine as explained below. (I have but it is dangerous!). A dentist explained that ultrasonic cleaning of teeth also liberates pure oxygen and ozone locally to the Ultrasonic probe, which in turn sterilises the bacteria in that locality. So the Ultrasonic bath does the same thing.
If you use flamable hydrocarbons in the Ultrasonic cleaning tank, or in a jar/tin in the water in the Ultrasonic tank, then actually the vibration will cause vapour of the lightest, most flammable compounds to be derived, with a risk of ignition at a very low flash point. Depending on what you use, there may also be some oxygen or ozone liberated, so the vapour can be explosive! If it goes "woof" you won't see the flash, and may lose more than your eyelashes! (Just like the oxygen-hydrogen mix liberated from lead-acid batteries when being re-charged).
Take care my friends....
K2