I assume you are concerned with oil in the open air, as opposed to being enclosed in a sealed space. It has certain volatile components that will evaporate. (That's why it has a smell.) The remainder will get thicker over time as the volatile components disappear. I suspect it may not evaporate completely--the way water does, for example. It will probably leave behind a varnish or sludge. Heat or light will speed the process.
Engine oil won't evaporate, however it can "blow" out if the crankcase is'nt vented properly such as a failing or plugged up pcv valve. Also some engines "burn" or use oil with worn rings ect.