What is Cutting Oil?

Reading another thread on here, it was recommended to use "cutting oil" for drilling steel. I'm sure there is a specific oil made for this use, but is it really necessary, or can just plain motor oil or WD40 be used? I usually dont use anything if the metal is less than 1/8" thick, but the few times I had to drill in thicker metal, I just spray WD40 a couple times a minute, or have squirted some used motor oil from a pump oiler can. Is there any reason to spend money on a special cutting oil? If so, where does a person get it? And what makes it different than plain oil?
Jakester
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On Mon, 19 Nov 2007 00:16:52 -0600, snipped-for-privacy@hotmail.com wrote:

WD40 doesn't work worth a darn. I tried it once when I ran out of cutting oil. One of the most common uses of cutting oil is in a pipe threading machine. Go to a plumbing supply store and purchase some. It will keep the edge of the cutting tool sharp as it keeps it cool and reduces the friction during cutting.
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Cutting oil is different in that it contains EP additived: http://en.wikipedia.org/wiki/EP_additive
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