Is there any important reason not to use the clear version of Rigid threading oil for drilling/threading on my mill and/or lathe? I do not use flood coolant very often so most of the time I'm using a brush to apply to the work
If you're speaking of the thick oil that Rigid sells for threading pipes, manually or by machine, it's not very good for general machining. It's meant for use at very low speeds and at the highest pressures. It's too thick.
You'll do much better with a lighter oil made for machining. I use mostly plain lard oil, which I've used for over 30 years, but there are better oils. Look for a "straight" (also called "neat") oil if you're applying it with a brush. Soluble (water-miscable) oils are not made for that, although they'll work reasonably well. They're formulated for cooling and are not very good lubricants. I was given some by a manufacturer of the stuff (Master Chemical), who told me to mix it very rich for use on my old South Bend, and it did the job. But straight oils are better, IMO.
The hard part is finding them in small quantities. DoAll, Acculube, Castrol, and several other companies make them. Petroleum-based, lard-based, or vegetable-oil-based probably doesn't make much difference.
If you're tapping by hand, use one of the cutting products made for that -- Tap Magic, etc. They also sell cutting oils in small quantities.
When you say "lard oil", are you talking about the white stuff from the gro cery store that used to me used for frying? Never messed with it but always figured it was a solid like Crisco. I would use the material in question f or drilling large holes in steel mostly. I have other stuff that I use for tapping (TapMagic) and machining Tri-cool or WD40). Have used kerosine in t he past but did not like the smell.
Here in the states, we have a Mexican restaurant named "Chipotle". All the y do is take a steamed piece of flat bread (named a tortilla) and wrap it a round already-cooked seasoned ground beef, mashed beans and onions and pepp ers, etc... and then serve it.
Another Mexican restaurant here, named "South of the Border" at first does the same but then they put oil or maybe Crisco on it and then bake the whol e thing in the oven for about 20, 25 or so minutes and then top it with che ese. . Then you cover it with lettuce, tomatoes, salsa, chillies, etc. I ts very good.