The clearances between metal engine parts can be as little is .0010″. The oil keep the metal parts from touching. But even with oil between the metal parts, the spinning creates heat. In addition to lubricating the parts, the oil also carries the heat away.
But oil also has several key additives. It has a detergent that keeps dirt suspended so it can move the dirt to the oil filter. Without detergent, the crud would either stay in place or settle at the bottom of the oil pan. Oil also has anti-friction additives that melt out of the oil if metal ever does touch, so it’s kind of “self healing.” Oil also contains anti-corrosion additives that prevent metal parts from corroding. Finally, it contains an anti-foaming agent to prevent bubbles when it’s whipped up by moving metal parts.
Regular oil is also made up of several different weights of oil. As you put on the miles, the lighter weight oils get burned off, leaving only the heavier oils. That’s why car makers want you to use synthetic–the molecules are the same size.
All of those additives wear out. That’s why you have to change your oil. Squeeze too many miles out of your oil and you will find black gooey sludge at the bottom of your engine. Since the anti-friction additives will be gone, you’ll see burned oil on all the engine parts (just like burned butter in a frying pan).
Any mechanic can instantly tell whether an owner has changed the oil regularly. Well kept engines are clean inside. Abused engines are coated with brown varnish.
Not changing your oil is simply the worst thing you can do to an engine. Not changing your coolant is right behind.
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