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How and Why to Check the Oil in Your Car's Engine


The yellow handle of an oil dipstick in the engine compartment of a car

You see that bright yellow handle in the picture? That's the handle of the oil dipstick; and it's a part of the car that every car owner and driver should get to know.

The oil dipstick is an old-fashioned, but effective way to check the oil level in your car. Most cars don't automatically monitor oil level. The "idiot light" on the instrument panel warns of low oil pressure, not low oil level. You have to check the oil level yourself using the dipstick.

Even on cars that do have low-oil detection light, the level at which the warning light will come on may be dangerously low. Something simple like going up or down a hill may result in the oil level at the intake siphon dropping low enough to cause oil starvation.

You have to understand that until the oil level drops to the point that the oil pump can no longer pull it out of the crankcase and pump it through the engine, the car will run just fine. Once it drops below that level, however, the engine will suffer catastrophic damage due to oil starvation. It only takes seconds for an unlubricated engine to seize.


How Often Should You Check the Oil in Your Car?

On a new or newish car, checking the oil level once a week is usually enough unless you notice that it is burning or losing oil. I make checking the oil part of a weekly ritual during which I also check the tires, tire pressure, belts, hoses, vacuum lines, battery cables, air filter, windshield washer fluid level, and brake fluid level.

I also check for mouse nests while I'm under the hood. Mice can do tremendous damage to a car, especially when they gnaw on the wiring. (Don't ask me how I know.) I've taken to hanging mouse repellant sachets under the hood to deter the little miscreants.

On an older car (or any car that you know burns oil), you should check your oil at least weekly, depending on how much oil the car typically burns or loses. I once had a car whose oil I had to check daily until I found and fixed the leak.

If your engine's dipstick is missing, you can purchase a new one from a dealership. Amazon also carries replacement oil dipsticks for many model cars.

What Can Happen if I Don't Check the Oil?

Well, that depends. If the oil level is okay, then nothing will happen. Your dipstick may get lonely, but no harm will be done as long as the oil level is in the safe range.

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If the oil level does drop below the minimum level, however, then you very likely will destroy the engine.

The most frequent kind of damage due to oil starvation is known as an engine "seizing," which basically means that two moving surfaces have generated enough heat that they have welded themselves to each other. This can happen to any moving parts, but some of the more common cases are piston rings seized to cylinders, connecting rods seized to crankshaft journals, and crankshafts seized to bearings The end result is the same: The engine suddenly stops and can no longer turn.

Some seized engines can be rebuilt. Others can't, in which case the engine will have to be replaced. It depends on the specific parts that have seized as well as any subsequent damage done by the sudden stop, such as crankshaft runout.

Either way, rebuilding or replacing the engine is a very expensive repair. On a used car, it may cost more than the car is worth; and even on a car that's under warranty, if the cause was owner neglect (as in, for example, not checking the oil), it probably won't be repaired under warranty.

The moral of the story is to check your oil regularly. Your car's engine's life depends on it.


How to Check the Oil in Your Car

Every car, truck, SUV, boat, and airplane engine I've ever met has had an oil dipstick. The handles are usually painted yellow or orange, as is anything else under the hood that the owner is responsible for checking or maintaining.

Motor oil on a dipstick showing the level at the high end of the safe oil level

Unless your car's owner's manual says otherwise, the oil should be checked when the engine is cold and the car is parked in a level area. Pull the dipstick, wipe any existing oil from the dipstick with a shop rag, reinsert the dipstick for a few seconds, and then pull it out again and hold it level.

At the opposite end of the dipstick from the handle will be some markings. In a nutshell, you want the oil to be between the upper and lower limits of those markings.

On cars sold in English-speaking nations, the letters "F" or "H" (for "Full" or "High" respectively), and an "L" (for "Low"), used to be the standard dipstick markings. Increasingly, however, pictographs or other markings that are not language-specific are being used to denote the upper and lower limits of the safe oil level range.

Some common international dipstick markings include crosshatching (the oil level should be within the crosshatched area), diagonal lines (same idea), or two holes or dots as in the picture on this page (the oil level should be between the two holes or dots).

Regardless of the markings used, the point is that if the oil level is between the upper and lower range of the marking, the oil level is safe to operate the engine.


When Should I Add Oil to My Car's Engine?

If the oil level is below the lower indicator, you must add oil to bring it within the safe range for engine operation.

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If the oil level is between the upper and lower limits, but not full, then you may add oil if you like: but never fill it higher than the upper limit of the safe range. Overfilling an engine's oil can cause aeration of the oil due to moving parts splashing in it. That can cause the oil to foam and not be able to properly lubricate the engine.

If the oil level is so low that it doesn't even show on the dipstick, then you shouldn't even start the engine until you have added oil. When the oil level is so low that it can't reach the dipstick, you are at or near that level at which oil starvation can occur and cause immediate and catastrophic damage to your engine. So do yourself and your car's engine a favor and always have a few extra cans of engine oil handy.

Although rare, there are oils that don't mix well with each other. When adding oil to an engine, it's therefore best to add the same oil as what's already in the engine. But if the oil level is below the lower limit of the safe range, and you don't have (or don't know) the oil that is in the engine, add any oil that falls within the car manufacturer's specifications. Oil incompatibility is fairly rare. Your engine is much more likely to suffer damage due to low oil.

When buying oil to do your own oil change, however, buy an extra quart (or liter) or two for top-offs. If you wind up not needing them, you'll have them for the next oil change.

The fact that engine oil has turned black is not in and of itself a reason to change the oil. One of the jobs of motor oil is to hold carbon and contaminants in suspension, so it's normal for oil to turn black. An experienced eye can sometimes tell when the consistency of oil doesn't seem right; but until you get to that level of expertise, just make sure you change your oil well within the manufacturer's recommended oil-change interval.

Motor oil being poured from a jug. Laboratory flasks representing synthetic motor oil. A drain pan being used to catch used oil being drained from a car engine. The open top of an engine coolant reservoir. Mechanic using a rooling creeper to work under a car. A hydraulic floor jack being used to raise a car. Three automobile engine oil filters. A mechanic using a wrench to work on a car engine.

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