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Some of the difference is an engine oil is intended to keep a film on the metal surfaces and when it is hot it will smoke and it does not cool as well as cutting oil.
"Lubricating oils have strong bonds between the atoms
which are depended upon to keep the molecules from shearing
in operation. Think lots of teeny ball bearings between
the metal surfaces. Strong ball bearings. Oil manufacturers
go to great lengths to keep their oil from shearing under
high pressure and temperature.
Now cutting fluids are an entirely different matter.
Cutting fluids are designed to carry heat away, and to
allow the molecules to shear into pieces so metal-on-metal
contact can happen, and cutting can occur. One way this
is done can be seen in my favorite cutting fluid, lard
oil. It's an animal fat with weak interatomic bonds.
But make no mistake, there are *huge* differences in
chemistry between lube oils and cutting fluids. Motor
oil is an especially bad choice for a cutting fluid.
Look around at any shop you see, how many professional
shops use motor oil as a cutting fluid, and how many
use a purpose-engineered material for this?"