In article ,
Ignoramus10071 wrote:

A metalworking fluid perform several functions.

1. It is a great conversation starter
2. It cools the cutter
3. It lubricates the contact area where the tip of the cutter removes
material, and therefore less heat is generated. (not the same as 2.)
4. It prevents microscopic welding of material to cutter, thus
improving surface finish and effective sharpness of the tip.
5. For high speed machining, high pressure stream of coolant removes
chips. (not something we encounter with manual machines)

I would also like to learn when straight oil is more appropriate than
soluble oil.

Depends on the straight oil, depends on the soluble oil, depends on the
material to be worked. There are only 5,000 variations.

For drilling and countersinking and tapping stainless steel, black
sulfur oil is hard to beat, although WS-5050 does work.

Joe Gwinn