We all know that our cars need engine oil in order to operate smoothly. The role of engine oil is to keep the moving parts of the engine lubricated, to protect them against rust corrosion, and -- with modern detergent oil additives -- to keep them free of sludge and general engine gunk.
But most of us also know some things about engine oil that aren't actually true. For instance, isn't it always necessary to change your oil every 4,828 kilometers? And when the color of your oil starts becoming dark, doesn't that mean that it's about to fill your engine with harmful sludge?
If you're conscientious about keeping your car in good running order, you probably worry from time to time that your oil has gotten dirty and is causing sludge to build up in your engine. So you pull the dipstick out and check the color of the oil at the tip. Chances are, it's starting to turn dark, no longer the light amber color that you saw on the stick when your oil was fresh. So now it's too dirty to use, right? It's depositing sludge in your engine and needs to be changed.
Wrong. In fact, just the opposite is true. If you're using a detergent engine oil (and most modern engine oils have detergent additives), the oil is working just the way it's supposed to, dispersing the tiny particles that can result in engine sludge and holding them in suspension in the oil itself so that they can't build up. That's why the oil appears darker, but this in no way impedes the oil from performing its normal functions of lubricating and protecting the metal surfaces inside the engine. Of course, the oil is limited in how many of these suspended particles it can contain and will eventually need to be changed when it becomes saturated, but use the oil change interval recommended by your car's manufacturer to decide when to change the oil, not the color of the oil on the stick.
This is true -- except that these "additives" have already been added before you buy the oil. Any reputable brand of motor oil will come with additives that improve its viscosity index -- the range of temperatures under which it flows properly through the engine -- and that give it detergent properties that keep your engine free of sludge. Most will also include rust retardants to prevent corrosion and chemicals to protect metallic surfaces.
With all these additives already in the oil, putting in more may actually dilute what's already there and lessen the oil's effectiveness. Check your car's manual to see if it has any special additive needs, but this is unlikely in anything except some of the most exotic high-performance engines.