One of the main jobs of engine oil is to transfer heat away from the engine. As a fluid, oil runs throughout the engine and bathes moving parts so that they don't scrape each other to bits from being in contact with one another.
Oil absorbs the heat from all that whirring, rotating and thrashing about that takes place in the engine. As the oil circulates back to the oil pan, it cools off so that it can repeat the circuit and remove more heat. The oil has to be at the right consistency to do this. If it's too thin or if it's thick and sludgy, it'll be inadequate to properly cool the engine for long.
An overheating engine, indicated by an engine temperature gauge on the instrument panel cluster, is one possible sign that oil has gotten too thick and is either caking or coking to put your engine in jeopardy.
You might even be able to eyeball it by shining a flashlight into the oil filler hole of your vehicle's crank case. If the situation is really bad, you might see black sludge coating the inside surface.