The easy answer is: "like dissolves in like." Motor oils, unlike the oils in paint, are designed to lubricate and carry heat away from engine parts, reducing friction and minimizing thermal fatigue. In order to function under these conditions they are designed not to evaporate or degrade with increasing temperatures. Since motor oils are used in internal combustion engines, by design they have a high flash point (temperature at which the vapors of a substance can be ignited). For hydrocarbons, the higher the flash-point the lower the vapor pressure (or tendency to evaporate).

In the art world there are many different kinds of paints, the order of fastest drying to slowest is: aerosol (spray paint), watercolors, latex, acrylic, and oil. Spray paints have volatile organic compounds that evaporate very fast causing the paint to dry quickly. Watercolors, latex, and acrylic paints are all water-based; watercolors are the thinnest of the three and so they dry the fastest. Oil-based paints are powder pigments (inorganic oxides: iron oxides, manganese oxides, chromium oxides, etc.) mixed with oil. Oil paints dry as a result of a combination of evaporation of the oils and absorption of the oils into the canvas or wood. In some cases the paint may dry, but never completely cure, because the oil does not fully evaporate. The rate of evaporation depends both on the oil(s) in the paint and environmental conditions. For example, linseed oil is a commonly used in oil-based paints because some of its components are volatile, allowing it to cure quickly. Linseed oil is intended for painting canvas and finishing wood products. In these applications, the oil is readily absorbed into the wood or canvas and slowly cures into a hard film, thus strengthening and protecting the wood. Linseed oil is derived from pressing ripe flax seeds and is sometimes mixed with volatile petroleum products so that the mixture evaporates more quickly.

My assumption is that the question refers to artists' oil paints, since the question states "motor oil" (hence the CSI reference) and not just oil. If you mix non-volatile, 30 weight motor oil with Linseed oil-based paint, the paint will mix well with the motor oil because the two oils are alike. If you then paint a metal railing with the paint, everything that would normally evaporate does and the motor oil, with the pigment particles, remains on the surface. Since the oil evaporates very slowly, or not at all, the paint never dries.

Incidently, as an experiment I mixed 30 weight motor oil with latex house paint (which actually has no latex in it) just to see what would happen. In short, the bucket of paint that I mixed and left sitting open to the air (for over a week) has not yet dried, like it normally would have. When I painted a piece of metal with this oil and paint mixture, it did not dry in the same way as the control (paint with no oil). The "dried" paint felt tacky, gooey, and slimey. The paint did not stick to the metal it was painted on but it did not feel like it was wet either, it just became a mass of oily, rubbery, colored goo. So the paint was dry, in the sense that it did not transfer to my hand, but it is no longer usable as paint. The explanation for this result is complicated, and can therefore be reserved for another article.