Part of routine vehicle maintenance includes periodic checks of the oil level. Every other fuel fill-up should be adequate. That way, you will not allow your engine to run low on oil. Also, if you have your oil changed regularly, it will remain in good shape and protect your engine.
But if you neglect oil changes, your oil will break down and eventually turn to sludge. There should not be any warning signs that you need an oil change (other than a maintenance reminder on your dashboard or calendar). Signs of neglect might include a ticking sound coming from your engine. As problems worsen, you might hear a deeper knocking sound. A burnt oil smell and smoke from the exhaust can also be signs set off by old, dirty oil. If you notice any of these, you have gone beyond the useful life of your oil.
Some people believe that their oil is bad simply because it appears dark black on the dipstick. While dirty oil certainly can look black in color, so can perfectly good oil. Soot that occurs naturally as a byproduct of combustion will stain your oil black, but it does not harm your engine. True, fresh oil is a translucent honey color, but it does not take too long to turn black. The only way to know if your oil is still good is to submit a sample for chemical testing… or to change it when recommended.